With an increase in the specific capacity of internal combustion engines, the amount of heat that needs to be removed from heated nodes and parts is increasing. Efficiency modern Systems Cooling and method of increasing the intensity of heat transfer practically reached its limit. The purpose of this work is to study the innovative cooling fluids for the cooling systems of heat power devices based on two-phase systems consisting of the base medium (water) and nanoparticles. One of the methods for measuring the thermal conductivity of the fluid called 3Ω-hot-Wire is considered. The results of measuring the coefficient of thermal conductivity of nanofins based on graphene oxide at various concentrations of the latter are presented. It has been established that when using 1.25% graphene, the thermal conductivity coefficient has increased by 70%.
thermal conductivity
coefficient of thermal conductivity
grefen oxide
nanosity
cooling system
test stand
1. Osipova V.A. Experimental study of heat exchange processes: studies. Handbook for universities. - 3rd ed., Pererab. and add. - M.: Energia, 1979. - 320 p.
2. Heat transfer /V.P. Isachenko, V.A. Osipova, A.S. Sukomel - M.: Energia, 1975. - 488 p.
3. Anomalously Increased Effective Thermal Conductivities of Ethylene Glycol-Based Nanofluids Containing Copper Nanoparticles / J.A. Eastman, S.S. Choi, S. Li, W. Yu, L.J. Thompson Appl. Phys. Lett. 78,718; 2001.
4. THERMAL CONDUCTIVITY MEASUREMENTS USING THE 3-OMEGA TECHNIQUE: Application to Power Harvesting Microsystems / David de Koninck; Theses of Master of Engineering, McGill University, Montréal, Canada, 2008. - 106 p.
5. THERMAL CONDUCTIVITY MEASUREMENT / W.A. Wakeham, M.J. Assael 1999 by CRC Press LLC.
It is known that with current trends in increasing the specific capacity of internal combustion engines, as well as to higher speeds and smaller sizes for microelectronic devices, the amount of heat that needs to be removed from heated nodes and parts is constantly increasing. The use of various heat-conducting fluids for heat removal is one of the most common and effective ways. The effectiveness of modern designs of cooling devices, like conventional way Increase the intensity of heat transfer, almost reached its limit. It is known that ordinary cooling fluids (water, oil, glycols, fluorocarbons) have a sufficiently low thermal conductivity (Table 1), which is a limiting factor in modern designs of cooling systems. To increase their thermal conductivity, it is possible to create a multiphase (minimum two-phase) dispersed medium, where the dispersion role particles with a significantly large thermal conductivity coefficient than the base fluid. Maxwell in 1881 suggested adding solid particles with high thermal conductivity to the basic heat-conducting coolant liquid.
The idea is to mix metal materials, such as silver, copper, iron, and non-metallic materials, such as alumina, Cuo, SiC and carbon tubes with higher thermal conductivity compared with the basic heat-conducting fluid with a smaller thermal conductivity coefficient. Original solid particles (such as silver, copper, iron, carbon tubes with higher thermal conductivity compared to base fluid) micron and even millimeter dimensions were mixed with basic liquids to obtain suspensions. The large size of the particles used and the difficulties in the production of nanoscale particles became limiting factors in the application of such suspensions. This problem was solved by the work of employees of the Arizonian National Laboratory S. Choi and J. Eastman, which conducted experiments with metal particles of nanometer sizes. They combined various metal nanoparticles and nanoparticles of metal oxides with various liquids and received very interesting results. These suspensions of nanostructured materials were named "nanofilaments".
Table 1
Comparison of thermal conductivity thermal conductivity coefficients
In order to develop modern innovative cooling fluids for cooling systems of high-phostened heat power devices, we covered two-phase systems consisting of a base environment (water, ethylene glycol, oil, etc.) and nanoparticles, i.e. Particles with characteristic dimensions from 1 to 100 nm. An important feature of nanodiality is that even when adding a small amount of nanoparticles, they show a serious increase in thermal conductivity (sometimes more than 10 times). Moreover, the increase in the thermal conductivity of nanofitting depends on temperature - with increasing temperature increases the increase in the thermal conductivity coefficient.
When creating such nanodities, which are a two-phase system, a reliable and fairly accurate method of measuring the coefficient of thermal conductivity are needed.
We examined different methods for measuring the thermal conductivity coefficient for liquids. As a result of the analysis, the "3Ω-wired" method was selected for measuring the thermal conductivity of nanodities with sufficiently high accuracy.
The "3Ω-wired" method is used to simultaneously measure the thermal conductivity and the temperature of the materials. It is based on measuring the increase in temperature, depending on the time in the heat source, that is, hot wire, which is immersed in fluid for testing. Metal wire simultaneously serves as an electric resistance heater and the resistance thermometer. Metal wire are extremely small in diameter (several dozen μm). Increasing the temperature of the wire reaches usually 10 ° C and the effect of convection can be neglected.
Metal wire Length L and R radius, weighted in the liquid, acts as a heater and the resistance thermometer, as shown in Fig. one.
Fig. 1. Installation scheme of the method "3Ω hot wire»To measure the thermal conductivity of the liquid
The essence of the method of determining the coefficient of thermal conductivity is as follows. AC current flows through a metal wire (heater). AC characteristic is determined by the equation
where i 0 is the amplitude of the variable sinusoidal current; ω - current frequency; T - Time.
A variable current flows through the wire, acting as a heater. In accordance with the law of Joule - Lenz, the amount of heat released during the passage of the electric current conductor is determined:
and is a superposition of the DC source and 2Ω modulated heat source,
where R e is electrical resistance Metal wire under the experimental conditions, and it is a function of temperature.
The above thermal power generates a temperature change in the heater, which is also the superposition of the DC components and the 2Ω components 2Ω:
where Δt dc is the amplitude of temperature change under the action of DC; Δt 2ω - amplitude of temperature change under the action of alternating current; φ is a phase shift induced by heating sample mass.
The electrical resistance of the wire depends on the temperature and this is 2Ω the AC component in the wire resistance:
where C RT is the temperature coefficient of resistance for the metal wire; R E0 is the reference resistance of the heater at a temperature T 0.
Usually t 0 is the temperature of the volume sample.
Voltage on the metal wire can be obtained as
(6)
In equation (6), the voltage on the wire contains: voltage drop due to the resistance of the DC of the wire at 1Ω and two new components proportional to the temperature increase in the wire at 3Ω and at 1Ω. 3Ω Voltage component 
It can be removed using an amplifier, and then used to output the amplitude of temperature change at 2Ω:
The frequency dependence of the temperature change ΔT 2Ω is obtained by changing the frequency of the AC at a constant voltage V 1ω. At the same time, the dependence of the temperature change Δt 2Ω on the frequency can be approximated as
where α F is the temperature coefficient; k f - the coefficient of thermal conductivity of the base fluid; η - constant.
A change in temperature at a frequency of 2Ω in a metal wire can be removed using the frequency voltage component 3Ω, as shown in equation (8). The thermal conductivity coefficient of the fluid K f is determined by the slope of 2Ω changes in the temperature of the metal wire relative to the frequency Ω,
(9)
where p is the power used; Ω is the frequency of the applied electric current; L is the length of the metal wire; Δt 2ω is the amplitude of temperature change at 2Ω in the metal wire.
The 3Ω wire method has several advantages over the traditional hot wire method:
1) Temperature oscillations can be sufficiently small (below 1K, compared with approximately 5K for the hot wire method) in the fluid under study to preserve the constant properties of the fluid;
2) background noise, such as temperature change, have a much smaller impact on the measurement results.
These advantages make this method ideally suitable for measuring the temperature dependence of the coefficient of thermal conductivity of nanipiles.
Installation for measuring the thermal conductivity coefficient includes the following components: Winston Bridge; signal generator; spectrum analyzer; oscilloscope.
Winston Bridge is a diagram used to compare the unknown resistance R x with a known resistance R 0. The bridge circuit is shown in Fig. 2. Four shoulders of the Winston AV, Sun bridge, ad and ds are resistances RX, R0, R1 and R2, respectively. The Diagonal VD includes a galvanometer, and the power supply is connected to the AC diagonal.
If you specify the values \u200b\u200bof the resistances R1 and R2 appropriately, it is possible to achieve equality of the potentials of points in and d: φ B \u003d φ D. In this case, the current via the galvanometer will not go, that is, I g \u003d 0. Under these conditions, the bridge will be balanced, And you can find an unknown resistance RX. To do this, we use the rules of Kirchhoff for branched chains. Using the first and second Rules of Kirchhoff, we get
R x \u003d R 0 · R 1 / R 2.
The accuracy in determining the R x the specified method is largely depends on the selection of resist R 1 and R 2. The greatest accuracy is achieved at R 1 ≈ R 2.
Signal generator acts as an electrical oscillation source in the range of 0.01 Hz - 2 MHz with high accuracy (with a discrete after 0.01 Hz). M3-110 signal generator brand.
Fig. 2. Winston Bridge Scheme
The spectrum analyzer is intended to release the 3Ω component of the spectrum. Before starting work, the spectrum analyzer was tested for compliance with the voltage of the third harmonic. To do this, an input of the spectrum analyzer is given from the generator G3-110 and in parallel to the broadband digital voltmeter. The effective value of the amplitude of the voltage was compared on the spectrum analyzer and voltmeter. The discrepancy of the values \u200b\u200bwas 2%. The spectrum analyzer calibration was also performed on an internal test of the device, at a frequency of 10 kHz. The value of the signal at the carrier frequency was 80 mV.
C1-114 / 1 oscilloscope is designed to study the form of electrical signals.
Before starting the study, the heater (wire) must be placed in the studied sample of the liquid. Wire should not touch the walls of the vessel. Next, the frequency is scanned in the range from 100 to 1600 Hz. On the spectrum analyzer, the value of the signal 1, 2, 3 of the harmonics in automatic mode is recorded at the frequency studied.
To measure the amplitude of the current force, used in a chain resistor resistor ~ 0.47 ohms. The value should be so that it does not exceed the value of the measuring shoulder of about 1 ohms. Using the oscilloscope was the voltage U. Knowing R and U, found the amplitude of the current strength I 0. To calculate the applied power, the voltage is measured in the chain.
Initially, a wide frequency range is investigated. The narrower frequency region is determined, where the linearity of the graph is highest. Then in the selected frequency domain, measurement is made with a smaller step of the frequency.
In tab. 2 The results of measuring the coefficient of thermal conductivity of the nanity, representing a 0.35% suspension of graphene oxide in the base fluid (water), with a copper insulated wire with a length of 19 cm, with a diameter of 100 microns, at a temperature of 26 ° C for frequency range of 780 ... 840 Hz.
In fig. 3 shows a general view of the stand for measuring the coefficient of thermal conductivity of the fluid.
In tab. 3 shows the dependence of the coefficient of thermal conductivity of the graphene oxide suspension from its concentration in the fluid at a temperature of 26 ° C. Measurements of the coefficients of thermal conductivity of nanimios were carried out at different concentrations of graphene oxide from 0 to 1.25%.
table 2
Results of measuring the thermal conductivity coefficient
|
frequency range |
Circular frequency |
Tok Power |
Third harmonic voltage amplitude |
Temperature change |
Logarithm circular frequency |
Power |
Slope graphics |
Coefficient of thermal conductivity |
Fig. 3. General view of the stand for measuring the heat conduction coefficient of fluid
In tab. 3 also shows the values \u200b\u200bof thermal conductivity coefficients defined by Maxwell formula.
(10)
where k is the coefficient of thermal conductivity nanosiness; k f - the coefficient of thermal conductivity of the base fluid; k p - thermal conductivity coefficient of the dispersed phase (nanoparticles); φ is the magnitude of the volume phase of each of the phases of dispersions.
Table 3.
The thermal conductivity coefficient of graphene oxide suspension
The ratio of the coefficients of the thermal conductivity K Exp / k Theorette and K Exp / k Table. Water shown in Fig. four.
Such deviations of the experimental data from the predicted by the classic Maxwell equation, in our opinion, may be associated with the physical mechanisms of increasing the thermal conductivity of nanosity, namely:
At the expense of the Brownian movement of particles; The mixing of the liquid creates a micro-convective effect, thereby increasing the energy of heat transfer;
Heat transfer according to the percolation mechanism is predominantly along the cluster channels formed by the agglomeration of nanoparticles that permeate the entire structure of the solvent (conventional liquid);
The base fluid molecules form highly oriented layers around the nanoparticles, thus increasing the volume fraction of nanoparticles.
Fig. 4. Dependence of the ratio of thermal conductivity coefficients from the concentration of graphene oxide
The work was carried out with the involvement of the equipment of the center of collective use by the scientific equipment "Diagnostics of micro and nanostructures" with the financial support of the Ministry of Education and Science of the Russian Federation.
Reviewers:
Eparrein O.M., D.T.n., Professor, Director of the Yaroslavl branch of FGBOU VPO "Moscow state University ways of communication ", Yaroslavl;
Amirov I.I., D.F.-M.N., Researcher of the Yaroslavl branch of FSBUN "Physics and Technological Institute" Russian Academy Sciences, Yaroslavl.
The work went on the editor 28.07.2014.
Bibliographic reference
Zharov A.V., Savinsky N.G., Pavlov AA, Evdokimov A.N. Experimental method of measuring the thermal conductivity of nanofitness // Fundamental studies. - 2014. - № 8-6. - P. 1345-1350;URL: http://fundamental-research.ru/ru/article/view?id\u003d34766 (Date of handling: 02/01/2020). We bring to your attention the magazines publishing in the publishing house "Academy of Natural Science"
Federal Agency for Technical Regulation and Metrology
NATIONAL
STANDARD
Russian
Federation
Composites
Official edition
STSHDFTTFTSM
GOST R 57967-2017
Preface
1 prepared by the Federal State Unitary Enterprise "All-Russian Research Institute of Aviation Materials" together with the autonomous non-profit organization "Center for the rationing, standardization and classification of composites" with the participation of the association of legal entities "Union of manufacturers of composites" based on official translation into Russian language of the English-language version of the specified Paragraph 4 of the Standard, which was completed by TC 497
2 Submitted by the Technical Committee on Standardization of TC 497 "Composites, Designs and Products of them"
3 approved and enacted by order of the Federal Agency for Technical Regulation and Metrology of November 21, 2017 No. 1785-st
4 This standard is modified in relation to the ASTM E1225-13 standard "Standard test method for determining the thermal conductivity of solids by a comparative longitudinal-fenced heat flux method" (ASTM E122S-13 "Standard Test Method for Thermal Conductivity of Solids using the Guard ED-Comparative -LONGITUDINAL HEAT Flow Technique », Mod) by changing its structure to bring in accordance with the rules set in GOST 1.5-2001 (subsections 4.2 and 4.3).
This standard does not include paragraphs 5. 12. Subparagraphs 1.2, 1.3 of the applied ASTM standard. which are inappropriate to apply in Russian national standardization due to their redundancy.
These items and subparagraphs not included in the main part of this Standard are given in the additional appendix yes.
The name of this standard has been changed relative to the name of the specified ASTM standard to bring in accordance with GOST R 1.5-2012 (subsection 3.5).
Comparison of the structure of this Standard with the structure of the specified ASTM standard is given in the additional application of dB.
Information on the compliance of the reference National Standard Standard ASTM. Used as a reference in the applied ASTM standard. shown in the additional application of DV
5 introduced for the first time
The rules for applying this standard are established in Article 26 Federal Law from June 29, 2015 N9 162-FZ "On standardization in Russian Federation" Information about the changes to this standard is published by the E-annual (as of January 1), the information indicators "National Standards", and the official text of the changes and the floor of the launch of the monthly information indicator "National Standards". In case of revision (replacement) or the cancellation of this standard, the appropriate notification will be published in the nearest issue of the monthly information indicator "National Standards". Relevant information. Notification and texts are also posted in the public information system - on the official website of the Federal Agency for Technical Regulation and Metrology on the Internet ()
© Stamartartinform. 2017.
This standard cannot be fully or partially reproduced, is replicated and distributed as an official publication without the permission of the Federal Agency for Technical Regulation and Metrology
GOST R 57967-2017
1 area of \u200b\u200buse............................................... ..................one
3 Terms, definitions and designations ............................................ .......one
4 Essence of the method ............................................... ..................... 2.
5 Equipment and materials ................................................ .............four
6 Preparation for testing ............................................. .......eleven
7 Testing ............................................................. ...............12
8 Processing test results ................................................ .......13
9 Test Protocol ................................................. ..................13
Appendix Yes (Reference) Original text of not included structural elements
aSTM standard applied ........................................... 15
Application dB (reference) comparison of the structure of this standard with the structure
aSTM standard applied in it ...................................... 18
Appendix DV (reference) information on the compliance of the reference national standard ASTM standard. Used as a reference in the applied ASTM standard ............................................ .............nineteen
GOST R 57967-2017
National Standard of the Russian Federation
Composites
Determination of thermal conductivity of solid bodies by stationary one-dimensional heat flux with a security heater
Composites. Determination Of Thermal Conductivity of Sohds by Stationary One-Dimensional Heat Flow
with a Guard Heater Technique
Date of introduction - 2018-06-01
1 area of \u200b\u200buse
1.1 This standard establishes the determination of the thermal conductivity of homogeneous opaque solid polymer, ceramic and metal composites using a stationary one-dimensional heat flux with a security heater.
1.2 This standard is intended for use in testing materials having affective thermal conductivity in the range from 0.2 to 200 W / (M-K) in the temperature range from 90 to 1300 K.
1.3 This standard can also be applied when testing materials having efficient thermal conductivity outside of the specified ranges with lower accuracy.
2 Regulatory references
This standard uses regulatory references to the following standards:
GOST 2769 Surface roughness. Parameters and characteristics
GOST R 8.585 State system for ensuring unity of measurements. Thermocouples. Nominal static conversion characteristics
Note - When using this standard, it is advisable to check the action of reference standards in the public information system - on the official website of the Federal Agency for Technical Regulation and Metrology on the Internet or on the National Standards Annual Information Signal, which is published as of January 1 of the current year, and on the issues of the monthly information pointer "National Standards" for the current year. If the reference standard is replaced, to which the undated link is given, it is recommended to use the current version of this standard, taking into account all changes made to this version. If the reference standard is replaced by a dated reference, it is recommended to use the version of this standard with the above-mentioned approval (adoption). If, after approval of this standard in the reference standard, which is given dated dated, the change is made, affecting the provider to which the link is given, this provision is recommended to be applied without taking into account this change. If the reference standard is canceled without replacement, the position in which the reference is given to it is recommended to be applied in a portion that does not affect this link.
3 Terms, Definitions and Designations
3.1 This standard applies the following terms with the corresponding definitions:
3.1.1 Thermal conductivity / .. W / (M K): the ratio of the density of the heat flux under stationary conditions through the unit of the area to the unit of temperature gradient e direction perpendicular to the surface.
Official edition
GOST R 57967-2017
3.1.2 Conducting thermal conductivity: if there are other ways to transfer heat through the Mate * Rial, except thermal conductivity, the results of measurements made under the present method of testing. represent seeming or efficient thermal conductivity.
3.2 8 This standard applies the following notation:
3.2.1 x M (t), W / (M K) - thermal conductivity of reference samples depending on temperature.
3.2.2 ECI, W / (M K) is the thermal conductivity of the upper reference sample.
3.2.3 xjj '. 8T / (M K) is the thermal conductivity of the lower reference sample.
3.2.4 EDT), W / (M K) - the thermal conductivity of the test sample adjusted to the heat exchange in non * crazy.
3.2.5 x "$ (T), W / (M K) - the thermal conductivity of the test sample, calculated without taking into account the amendment for heat exchange.
3.2.6\u003e U (7), W / (M K) - thermal conductivity of insulation depending on temperature.
3.2.7 g, K - absolute temperature.
3.2.8 z, M - distance measured from the upper end of the package.
3.2.9 /, M - length of the test sample.
3.2.10 g (, K - temperature at z r
3.2.11 q ", W / m 2 - thermal flow per unit area.
3.2.12 SKH, etc. - deviations of X. G. DR.
3.2.13 g a, m - radius of the test sample.
3.2.14 g, M - the inner radius of the security shell.
3.2.15 F 9 (z), K is the temperature of the security shell, depending on the distance Z.
4 Essence of the method
4.1 General scheme of a stationary one-dimensional heat flux using OH * An early heater is shown in Figure 1. Test sample with an unknown thermal conductivity X s. having an estimated specific thermal conductivity x s // s. Install under load between two reference samples with thermal conductivity of X M, having the same area cross section and specific thermal conductivity x ^ // ^. The design is a package consisting of a disc heater with a test sample and reference samples on each side between the heater and the heat sink. In the test packet, a temperature gradient is created, heat losses are minimized by using a longitudinal security heater having an approximation of the same temperature gradient. After each sample, approximately half of the energy flows. 8 equilibrium condition The thermal conductivity coefficient is determined based on the measured grades of the temperature of the test sample and the corresponding reference samples and the thermal conductivity of reference materials.
4.2 apply power to the package to ensure good contact between the samples. The package is surrounded by insulating material with thermal conductivity insulation enclosed in a security unit with a radius of g 8, which is at temperatures T d (2). Set the temperature gradient in the package by maintaining the upper part at temperatures t t and the bottom at the temperature T in. Temperature T 9 (Z) is usually a linear temperature gradient approximately appropriate gradient installed in the test package. Can an isothermal security heater with a temperature t? (Z). equal to the average temperature of the test sample. It is not recommended to use the design of the measuring cell of the device without security heaters due to possible large thermal losses, especially at elevated temperatures. In the stationary state, temperature gradients along the plots are calculated based on the measured temperatures along two reference samples and the test sample. The value of x "s Excluding amendments to the heat exchange is calculated by the formula (the symbols are shown in Figure 2).
T 4 -G 3 2 U 2 -z, z e -z 5
where r, temperature at Z ,. K T 2 - temperature at z 2, K g 3 - temperature at z 3. TO
GOST R 57967-2017
G 4 - temperature at z 4. TO;
G 5 - temperature at z s. TO:
G - temperature at z e. TO:
Z, - coordinate of the 1st temperature sensor, m;
Zj - coordinate of the 2nd temperature sensor, m;
Z 3 - coordinate of the 3rd temperature sensor, m;
Z 4 - coordinate of the 4th temperature sensor, m;
Z 5 - coordinate of the 5th temperature sensor, m;
Z E - coordinate 6\u003e th temperature sensor, m.
Such a scheme is idealized, as it does not take into account the heat exchange between the packet and insulation at each point and the uniform heat transfer on each boundary of the separation samples and the test sample. The errors caused by these two assumptions can change much. Due to these two factors, restrictions on this test method should be provided. If required to achieve the necessary accuracy.
1 - temperature gradient in the security shell: 2 - temperature gradient in the package; 3 - thermocouple: 4 - clamp.
S - top heater. B - upper reference sample: 7 - lower reference sample, B - lower heater: B - refrigerator. 10 - Upper Secure Natreahel: And - Inzia Wild Heater
Figure 1 - Scheme of a typical tested package and security shell, showing the compliance of temperature gradients
GOST R 57967-2017
7
b.
Refrigerated нг.
Olya Oimshpram
Insulation; 2 - security heater. E - Metal or ceramic security shell: 4 - heater. S is a reference sample, b - test sample, x - approximate location of the thermocouple
Figure 2 - diagram of the method of one-dimensional stationary heat flux using a security heater indicating possible locations of temperature sensors
5 Equipment and materials
5.1 Reference samples
5.1.1 For reference samples, reference materials or standard materials with known thermal conductivity values \u200b\u200bshould be used. Table 1 shows some of the generally accepted reference materials. Figure 3 shows an approximate change\u003e. m with tempera * tour.
GOST R 57967-2017
Typlofoeodoost, IML ^ M-K)
Figure 3 - Reference values \u200b\u200bof thermal conductivity of reference materials
Note - Selected for reference samples The material must have the thermal conductivity closest to the thermal conductivity of the measured material.
5.1.2 Table 1 is not exhaustive, and other materials can be used as reference. The reference material and the source of the values \u200b\u200bof the X M must be specified in the test protocol.
Table 1 - Reference data Characteristics of reference materials
GOST R 57967-2017
End of Table 1.
Table 2 - thermal conductivity of electrolytic iron
|
Temperature. TO |
Thermal conductivity. W / (mk) |
GOST R 57967-2017
Table 3 - Tungsten thermal conductivity
|
Temperature, K. |
Thermal conductivity. 6T / (MK) |
GOST R 57967-2017
Table 4 - thermal conductivity of austenitic steel
|
Temperature. TO |
Thermal conductivity, W / (m k) |
GOST R 57967-2017
End of Table 4.
5.1.3 Requirements for any reference materials include the stability of properties in the entire operating temperature range, compatibility with other components of the measuring cell of the device, the ease of fastening the temperature sensor and the exactly known thermal conductivity. Since errors due to heat loss for a particular increase of K are proportional to the change in K and JK S, the reference material C) should be used for reference samples. m. The closest to\u003e. s.
5.1.4 If the thermal conductivity of the test sample K s is between the thermal conductivity values \u200b\u200bof the two reference materials, the reference material with a higher thermal conductivity to and is to use. To reduce the total temperature drop along the package.
5.2 Insulating materials
As insulating materials, powder, dispersed and fibrous materials are used to reduce the radial heat flux into the ring space and heat loss along the package. It is necessary to take into account several factors when choosing insulation:
Insulation should be stable in the expected temperature range, have a low thermal conductivity value to and be easy to use;
Isolation should not pollute the components of the measuring cell of the device, such as temperature sensors, it should have low toxicity and not a bill of electrical out.
Usually use powders and solid particles, as they are easy to ravibly. Low density fibrous mats can be used.
5.3 Temperature sensors
5.3.1 On each reference sample, at least two temperature sensors and two on the test sample must be installed. If possible, reference samples and the test sample must contain three temperature sensors in each. Additional sensors are required to confirm the layer of temperature distribution along the package or detection of an error due to the non-therapist of the temperature sensor.
5.3.2 Temperature sensor type depends on the size of the measuring cell of the device, temperature range and ambient In the measuring cell of the instrument, determined by insulation, reference samples, test sample and gas. To measure the temperature, any sensor that has sufficient accuracy can be used, and the measuring cell of the device must be quite large so that the perturbation of the heat flux from the temperature sensors was insignificant. Typically used thermocouples. Them small sizes And the ease of fastening makes explicit advantages.
5.3.3 Thermocouples should be made of wire with a diameter of no more than 0.1 mm. For all cold spa, a constant temperature should be provided. This temperature is supported by a cooled suspension, a thermostat or electronic reference point compensation. All thermocouples should be made either from the calibrated wire or from wire, which was certified by the supplier to ensure the limits of the error indicated in GOST R 8.585.
5.3.4 The thermocouple methods are shown in Figure 4. Internal contacts can be obtained in metals and alloys by welding individual thermoelements to surfaces (Figure 4A). Spi the thermocouple, welded or with a kolkom can be rigidly attached with forging, cementing or welding in narrow grooves or small holes (Figures 4P. 4C and 4 5.3.5 In Figure 46, the thermocouple is located in the radial groove, and in Figure 4c, the thermocouple is pulled through the radial hole in the material. 8 case of using thermocouples in a protective shell or thermocouple, both thermoelement which is located in an electric insulator with two GOST R 57967-2017 holes, the thermocouple fastening can be used, shown in Figure 4D. In the last three cases, the thermocouple should be thermally connected to the solid surface with a suitable glue or high-temperature reader. 8se four procedures shown in Figure 4. should include hardening wires on surfaces, wire turns in isothermal zones, heat grounding of wires on a security casing or a combination of all three. 5.3.6 Since the inaccuracy of the temperature sensor leads to large errors. Special attention should be paid to the definition of the correct distance between the sensors and the calculation of a possible error as a result of any inaccuracy. b - internal cheese with separated thermoelements, welded to the test sample or reference samples so that the signal passes through the material. 6 - radial groove on a flat surface of the attachment of a bare wire or a thermocouple sensor with ceramic insulation; C is a small radial hole drilled through a test sample or reference samples, and uninsulated (it is allowed if the material is an electric insulator) or an insulated thermocouple, stretched through the hole: D - a small radial hole, drilled ■ test sample or reference samples, and thermocouple placed about the hole Figure 4 - Fastening the thermocouple Note - In all cases, thermoelements should be thermally hardened or thermally grounded to the security shell to minimize the measurement error due to the heat flux to or from hot spa. 5.4 Loading system 5.4.1 Test method requires uniform heat transfer across the boundary of the section of reference samples and the test sample, when the temperature sensors are located at a distance in the limits of the part of the partition. To do this, it is necessary to ensure a uniform contact GOST R 57967-2017 the tyal of the adjacent zones of reference samples and the test sample, which can be created by applying the axial load in combination with the conductive medium on the interface. It is not recommended to carry out measurements in a vacuum if it does not require a DDI protective goals. 5.4.2 When testing materials with low thermal conductivity, thin test samples are used, so temperature sensors must be installed close to the surface. In such cases, a very thin layer of high thermal conducting fluid, paste, soft metal foil or screen should be introduced at the interfaces. 5.4.3 In the design of the measuring instrument, items should be provided for overlapping and permanent loading a package to minimize interfacial resistance at the boundaries of the reference samples and the test sample. The load can be applied pneumatically, hydraulically, the action of the spring or the location of the cargo. The above load application mechanisms are constant when changing the package temperature. In some cases, the strength to compress the test sample may be so low that the applied force must be limited to the weight of the upper reference sample. In this case, special attention should be paid to the errors that can be caused by bad contact, for which the temperature sensors need to be located away from any perturbation of the heat flux at the interfaces. 5.5 Security shell 5.5.1 A package consisting of a test sample and reference samples must be enclosed in a protective shell with proper circular symmetry. The security shell can be metallic or ceramic, and its inner radius should be such that the ratio of g ^ g a was in the range from 2.0 to 3.5. The security shell must contain at least one security heater for adjusting the temperature profile of the shell. 5.5.2 The security shell must be designed and function in such a way that its surface temperature is either isothermal and approximately equal to the average temperature of the test sample, or have an approximate linear profile, coordinated at the upper and lower ends of the security shell with the corresponding positions of the Oder package. In each case, at least three temperature sensors should be installed on a security shell in pre-corrodinous points (see Figure 2) for measuring the temperature profile. 5.6 Measuring equipment 5.6.1 The combination of the temperature sensor and the measuring instrument used to measure the output signal of the sensor must be adequate to ensure the accuracy of temperature measurement ± 0.04 K and absolute error less than ± 0.5%. 5.6.2 Measuring equipment of the DDA of this method must maintain the desired temperature and measurement of all respective output voltages with an accuracy of the accuracy of temperature measurement accuracy temperature sensors. 6.1 Requirements for test samples 6.1.1 The test samples under investigators under this method are not limited to the candy geometry. Most preferably, the use of cylindrical or prismatic samples. The conductivity areas of the test sample and reference samples should be the same with an accuracy of 1% and any difference in the area should be taken into account when calculating the result. For a cylindrical configuration, the radii of the test sample and reference samples should be coordinated with an accuracy of ± 1%. And the radius of the test sample G A should be such that R B FR A is from 2.0 to 3.5. Each flat surface under test and the reference samples should be flat with a surface roughness not more than R a 32 in accordance with GOST 2789. And normal to each surface should be parallel to the axis of the sample with an accuracy of up to ± 10 min. Apply N and E - In some cases, this requirement is not necessary. For example, some devices can consist of reference samples and test samples with high values\u003e. M and\u003e. s. Where mistakes due to heat loss are insignificant for long sections. Such sections may have sufficient length, allow GOST R 57967-2017 it is to secure the temperature sensors at a sufficient distance from the contact places, thereby ensuring the uniformity of the heat flux. The length of the test sample must be selected on the basis of information about the radius and thermal conductivity. When). and higher than the thermal conductivity of stainless steel, long test samples can be used with a length of 0g A "1. Such long test samples can use long distances between temperature sensors, and this reduces the error obtained due to inaccuracies in the location of the sensor. When). m below than the thermal conductivity of stainless steel, the length of the test sample must be reduced, since the measurement error due to heat loss becomes too large. 6.1.2 Unless otherwise established in the regulatory document or technical documentation for the material. For testing use one test sample. 6.2 Equipment Setup 6.2.1 Calibration and equipment calibration is performed in the following cases: After assembling the equipment: If the ratio of x m to x s is less than 0.3. or more than 3. and choose the thermal conductivity values \u200b\u200bis not possible; If the shape of the test sample is a complex or test sample small: If there were changes in geometrical parameters measuring cell instrument; If it was decided to use the materials of reference samples or isolation other than those shown in sections 6.3 and 6.4: If the equipment has previously functioned to a sufficiently high temperature at which the properties of the components may change, such as. For example, the sensitivity of the thermocouple. 6.2.2 The specified checks should be carried out by comparing at least two reference materials as follows: Select the reference material, the thermal conductivity of which is closest to the intended thermal conductivity of the test sample: The thermal conductivity of the X test sample made from the reference material is measured using reference samples made from another reference material, which is X. The closest to the value of the test sample. For example, check can be carried out on the satal sample. Using reference samples made of stainless steel. If the measured thermal conductivity of the sample is not consistent with the value from Table 1 after applying the heat exchange amendment, it is necessary to determine the sources of errors. 7.1 Choose reference samples so that their thermal conductivity is the same order of magnitude that is expected for the test sample. After equipping the necessary reference samples with temperature sensors and their installations in the measuring cell, the test sample is equipped with similar means. The test sample is inserted into the package so that it is placed between the reference samples and in contact with the adjacent reference samples at least 99% of the area of \u200b\u200beach surface. To reduce surface resistance, a soft foil or other contact medium can be used. If the measuring cell must be protected from oxidation during the test, or if the measurement requires a certain gas or gas pressure to control X / T, the measuring cell is filled and is purged by a mounted pressure gas. To load the package, the power must be applied to reduce the effects of uneven thermal resistance at the border of the phase partition. 7.2 Includes the upper and lower heaters at both ends of the package and adjust until then. While the temperature difference between points 2, and Zj. Z3 and Z 4. And also z s and 2 ^ will not be greater than the 200-fold error of the temperature sensor, but not more than 30 K. and the test sample will not be at the average temperature required for measurement. Despite. As the exact temperature profile along the security shell is not required for 3. The power of the security heaters is adjusted to those LOR, while the temperature profile is along the shell T G / s pipe length, and side, employees to eliminate heat leakage through the ends of the device (pipes). The pipe is installed on suspensions or on stands at a distance of 1.5-2 m from the floor, walls and ceiling of the room. The temperature of the pipe and the surface of the test material is measured by thermocouples. When testing, it is necessary to adjust the power of electricity consumed by security sections to eliminate the temperature difference between the working and security section The electricity consumption of the working heater can be measured as a wattmeter and a separate voltmeter and ammeter. Thermal conductivity of material, W / (m ■ ° C), X -_____ D. Where D.
- the outer diameter of the test product, m; D.
-
The inner diameter of the test material, m; - temperature on the surface of the pipe, ° C; T.
2
- temperature on the outer surface of the test product, ° C; I - length of the working section of the heater, m. In addition to thermal conductivity, on this device, you can measure the magnitude of the heat flux in the thermal insulation structure made from one or another thermal insulation material. Thermal stream (W / m2) Determination of thermal conductivity based on the methods of nonstationary heat flow (dynamic measurement methods). Methods based on the
Measuring non-stationary heat fluxes (methods of dynamic measurements), recently all wider are used to determine the thermophysical values. The advantage of these methods is not only a comparative speed of experiments, but and A greater amount of information received in one experience. Here, one more time is added to other parameters of the monitored process. Due to this, only the dynamic methods allow to obtain the thermophysical characteristics of materials such as thermal conductivity, heat capacity, temperature, cooling pace (heating) according to the results of one experiment Currently, there are a large number of methods and devices for measuring dynamic temperatures and heat fluxes. However, they all demand know Therefore, methods based on measuring stationary heat fluxes differ from the methods under consideration significantly greater identity between measurement results and the true values \u200b\u200bof the measured thermal values. Perfections about B and E and E dynamic measurement methods goes in three directions. First, it is the development of the methods for analyzing errors and the introduction of amendments to the measurement results. Secondly, the development of automatic corrective devices to compensate for dynamic errors. Consider the two methods most common in the USSR based on measuring non-stationary heat flux. 1. Method of regular thermal regime with bikal - rimeter. When applying this method, various types of biclorimeters design can be used. Consider one of them - a small-sized flat bicalry - meter of type MPB-64-1 (Fig. 25), which is designed The MPB-64-1 device is a cylindrical shape of a plug-in shell (body) with an internal diameter of 105 mm, in center which is built in the core with mounted in It is a heater and battery of differential thermocouples. The device is made of duralumin Mark D16T. The thermobatrum of differential thermocouples bicked - rimeter is equipped with copper-copper thermocouples, the diameter of the electrodes of which is 0.2 mm. The ends of the turns of the thermobatars are removed on the brass petals of the rings of fiberglass, impregnated with BF-2 glue, and then through the wires to the fork. Heating element made ofNichrome wire with a diameter of 0.1 mm, cherished on a circular plate with a chicken BF-2 glass Fabrics. The ends of the wire of the heating element, as well as the ends of the thermobatar wire, are displayed on the brass rings and further, through the plug, to the power source. The heating element can be powered by an alternating current of 127 V. The device is sealed due to the seal from vacuum rubber, laid between the housing and the lids, as well as the gland pad (penkovo-sucrony) between the handle, the bobbish and the housing. Thermocouples, heater and their conclusions should be well isolated from the housing. The dimensions of the test samples should not exceed in diameter 104
mm and thick-16 mm. On the device simultaneously produce a test of two twin samples. The operation of the device is based in the following principle. The process of cooling the solid heated to temperature
T.°
and placed on Wednesday with Temperature ©<Ґ при весьма большой теплопередаче (а) от тела toThe medium ("-\u003e - 00) and at a constant temperature of this medium (0 \u003d const), is divided into three stages. 1. Temperature distribution in The body is at first a random character, that is, there is a disordered thermal mode. 2. Over time, cooling becomes ordered, i.e. the regular regime comes, in which Q. -
AUE .- "1 Where © is an elevated temperature in some point of the body; U - some point coordinate function; E-foundation of natural logarithms; T - time from the beginning of the cooling of the body; t - the pace of cooling; A is a constant device depending on the initial conditions. 3. After regular cooling mode is characterized by the onset of thermal body equilibrium with the environment. Temp Cooling t after differentiation of expression By T. in coordinates IN.IN-T. It is expressed as follows: Where BUT
and IN -
Constants of the device; FROM
- The total heat capacity of the test material equal to the product of the specific heat capacity of the material on its mass, J / (kg-° C); T - the rate of cooling, 1 / h. The test is carried out as follows. After placing the samples in the device, the device cover is tightly pressed to the housing using a nut with a knurling. The device is lowered into a thermostat with a stirrer, for example, a thermostat of TC-16, filled with water temperature, then connect the thermoplace of differential thermocouples to the galvanometer. The device is kept in a thermostat to level the temperature of the outer and inner surfaces of the samples of the test material, which is recorded by the galvanometer. After that, the core heater includes. The core is heated to a temperature greater than 30-40 ° water temperature in the thermostat, and then turn off the heater. When the galvanometer's arrow returns to the scale of the scale, record the discretion of the galvanometer decreasing in time. Total record 8-10 points. In the coordinate system 1P0-T, a graph is built, which should have the kind of a straight line crossing at some points of the abscissa axis and ordinate. Then calculate the tangent angle of inclination of the resulting direct, which expresses the value of the process of cooling the material: __
In 6t.
-
IN.
O2.
__
6
02
TIU - - J. T2 - TJ 12 - "EL Where Bi and 02 are the corresponding ordinates for TI and T2 time. Experience repeat again and once again determine the rate of cooling. If the discrepancy in the values \u200b\u200bof the cooling rate calculated during the first and second experiments, less than 5%, are limited to these two experiments. The average value of the cooling rate is determined by the results of two experiments and calculate the thermal conductivity of the material, W / (M * ° C) X \u003d (A + Yasure) / and. Example. The test material is a mineral wool mat on a phenolic binder with an average density in a dry state of 80 kg / m3. 1. Calculate the magnitude of the sample material placed in the device, Where the RP is a material placed in one cylindrical capacity of the device, kg;
VN.
- the volume of one cylindrical tank of the device equal to 140 cm3; PCP - average material density, g / cm3. 2. Determine composition
Bcyp.
,
Where IN
- a device constant, equal to 0.324; C is the specific heat capacity of the material equal to 0.8237 kJ / (kg-K). Then VSR \u003d.
=0,324 0,8237 0,0224 = 0,00598. 3. Results Observations for Cooling samples in the device in time we are in the table. 2. Discrepancies in the values \u200b\u200bof the cooling rate T and T2 are less than 5%, so repeat experiments can not be produced. 4. Calculate the average pace of cooling T \u003d (2.41 + 2,104) / 2 \u003d 2.072. Knowing all the necessary values, we count the thermal conductivity (0.0169 + 0.00598) 2.072 \u003d 0.047 W / (M-K) Or W / (m- ° C). At the same time, the average temperature of the samples was 303 to or 30 ° C. in formula 0,0169-l (the device constant). 2. Probe method. There are several varieties of the probe method for determining the heat pipe This method is as follows. Metal rod with a diameter of 5-6 mm (Fig. 26) and a length of about 100 mm are injected into the thickness of the hot thermal insulation material and with the help of an inside the rod Thermocouples determine the temperature. The temperature determination is made in two receptions: at the beginning of the experiment (at the time of the probe heating) and at the end, when the equilibrium state occurs and the increase in the temperature of the probe is terminated. The time between these two counts is measured using the stopwatch. h thermal conductivity material W / (M ° C), R.2CV. Where R. - Rod radius, m; FROM - specific heat capacity of the material from which the rod, KJ / (KGH HC) is made; V-volume rod, m3; T - time interval between temperature references, h; TX and U - temperatures at the time of the first and second samples, to or ° C. This method is very simple and allows you to quickly determine the thermal conductivity of the material both in laboratory and in production conditions. However, it is suitable only for a rough estimate of this indicator.
6 Preparation for testing
7 Testing
mi. Tests are carried out with a steady heat mode, at which the temperature on the surfaces of the pipe and insulating material is constant for 30 minutes.
Specific conditions and the introduction of amendments to the results obtained, since the process of measuring thermal values \u200b\u200bdiffer from the measurement of the values \u200b\u200bof other nature (mechanical, optical, electrical, acoustic, etc.) with its significant inertia.
To determine the thermal conductivity of semi-rigid, fibrous and bulk thermal insulation materials at room temperature.
Rum change in temperature at each point of the body obeys the exponential law:
The insulating materials differing from each other by the applicable devices and the principles of heating the probe. Consider one of these methods - the method of the cylindrical probe without an electric heater.
