Air humidity. Heat capacity and enthalpy of air
Atmospheric air is a mixture of dry air and water vapor (from 0.2% to 2.6%). Thus, the air can almost always be considered as humid.
The mechanical mixture of dry air and water vapor is called moist air or air/steam mixture. The maximum possible content of vaporous moisture in the air m a.s. temperature dependent t and pressure P mixtures. When it changes t And P the air can go from initially unsaturated to a state of saturation with water vapor, and then excess moisture will begin to fall out in the gas volume and on the enclosing surfaces in the form of fog, hoarfrost or snow.
The main parameters characterizing the state of humid air are: temperature, pressure, specific volume, moisture content, absolute and relative humidity, molecular weight, gas constant, heat capacity and enthalpy.
According to Dalton's law for gas mixtures wet air total pressure (P) is the sum of the partial pressures of dry air P c and water vapor P p: P \u003d P c + P p.
Similarly, the volume V and the mass m of moist air will be determined by the relations:
V \u003d V c + V p, m \u003d m c + m p.
Density And specific volume of humid air (v) defined:
Molecular weight of moist air:
where B is the barometric pressure.
Since the air humidity continuously increases during the drying process, and the amount of dry air in the vapor-air mixture remains constant, the drying process is judged by how the amount of water vapor changes per 1 kg of dry air, and all indicators of the vapor-air mixture (heat capacity, moisture content, enthalpy and etc.) refers to 1 kg of dry air in moist air.
d \u003d m p / m c, g / kg, or, X \u003d m p / m c.
Absolute air humidity- mass of steam in 1 m 3 of moist air. This value is numerically equal to .
Relative humidity - is the ratio of the absolute humidity of unsaturated air to the absolute humidity of saturated air under given conditions:
here , but more often the relative humidity is given as a percentage.
For the density of moist air, the relation is true:
Specific heat humid air:
c \u003d c c + c p ×d / 1000 \u003d c c + c p ×X, kJ / (kg × ° С),
where c c is the specific heat capacity of dry air, c c = 1.0;
c p - specific heat capacity of steam; with n = 1.8.
The heat capacity of dry air at constant pressure and small temperature ranges (up to 100 ° C) for approximate calculations can be considered constant, equal to 1.0048 kJ / (kg × ° C). For superheated steam, the average isobaric heat capacity at atmospheric pressure and low degrees of superheat can also be assumed to be constant and equal to 1.96 kJ/(kg×K).
Enthalpy (i) of humid air- this is one of its main parameters, which is widely used in the calculations of drying installations, mainly to determine the heat spent on the evaporation of moisture from the materials being dried. The enthalpy of moist air is related to one kilogram of dry air in a vapor-air mixture and is defined as the sum of the enthalpies of dry air and water vapor, that is
i \u003d i c + i p × X, kJ / kg.
When calculating the enthalpy of mixtures, the starting point of reference for the enthalpies of each of the components must be the same. For calculations of moist air, it can be assumed that the enthalpy of water is zero at 0 o C, then the enthalpy of dry air is also counted from 0 o C, that is, i in \u003d c in * t \u003d 1.0048t.
Russian Federation Protocol of the State Standard of the USSR
GSSSD 8-79 Liquid and gaseous air. Density, enthalpy, entropy and isobaric heat capacity at temperatures of 70-1500 K and pressures of 0.1-100 MPa
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STATE STANDARD REFERENCE DATA SERVICE
Standard Reference Data Tables
AIR LIQUID AND GAS. DENSITY, ENTHALPY, ENTROPY AND ISOBARIC HEAT CAPACITY AT TEMPERATURES 70-1500 K AND PRESSURES 0.1-100 MPa
Tables of Standard Reference Data
Liquid and gaseous air Density, enthalpy, entropy and isobaric heat capacity at temperatures from 70 to 1500 K and pressures from 0.1 to 100 MPa
DEVELOPED by the All-Union Scientific Research Institute of the Metrological Service, the Odessa Institute of Marine Engineers, the Moscow Order of Lenin Energy Institute
RECOMMENDED FOR APPROVAL by the Soviet National Committee for the Collection and Evaluation of Numerical Data in the Field of Science and Technology of the Presidium of the USSR Academy of Sciences; All-Union Research Center of the State Service for Standard Reference Data
APPROVED by the GSSSD expert commission consisting of:
cand. tech. Sciences N.E. Gnezdilova, Dr. tech. Sciences I.F. Golubeva, Dr. of Chem. Sciences L.V. Gurvich, Doctor of Engineering. Sciences V.A. Rabinovich, Doctor of Engineering. Sciences A.M.Siroty
PREPARED FOR APPROVAL by the All-Union Research Center of the State Service for Standard Reference Data
The use of standard reference data is mandatory in all sectors of the national economy
These tables contain the most important for practice values of density, enthalpy, entropy and isobaric heat capacity of liquid and gaseous air.
The tables are based on the following principles:
1. The equation of state, which reflects reliable experimental data on the , , -dependence with high accuracy, can provide a reliable calculation of caloric and acoustic properties from known thermodynamic relationships.
2. Averaging the coefficients of a large number of equations of state, which are equivalent in terms of the accuracy of describing the initial information, makes it possible to obtain an equation that reflects the entire thermodynamic surface (for a selected set of experimental data among equations of the accepted type). Such averaging makes it possible to estimate the possible random error in the calculated values of thermal, caloric and acoustic quantities, without taking into account the influence of the systematic error of the experimental , , -data and the error due to the choice of the form of the equation of state.
The averaged equation of state for liquid and gaseous air has the form
Where ; ; .
The equation is based on the most reliable experimental density values obtained in the works and covering the temperature range 65-873 K and pressures 0.01-228 MPa. The experimental data are described by an equation with a mean square error of 0.11%. The coefficients of the averaged equation of state were obtained as a result of processing a system of 53 equations that are equivalent in accuracy to the description of experimental data. In the calculations, the following values of the gas constant and critical parameters were taken: 287.1 J/(kg K); 132.5 K; 0.00316 m/kg.
Coefficients of the averaged air state equation:
Enthalpy, entropy and isobaric heat capacity were determined by the formulas
Where , , are the enthalpy, entropy and isochoric heat capacity in the ideal gas state. The values and are determined from the relations
Where and - enthalpy and entropy at temperature; - heat of sublimation at 0 K; - constant (in this work 0).
The value of the heat of sublimation of air was calculated on the basis of data on the heats of sublimation of its components and is equal to 253.4 kJ/kg Ar by volume). The values of enthalpy and entropy at a temperature of 100 K, which is an auxiliary reference point when integrating the equation for , are 3.48115 kJ/kg and 20.0824 kJ/(kg K), respectively.
The isobaric heat capacity in the ideal gas state is borrowed from the work and approximated by the polynomial
The root-mean-square error of approximation of the initial data in the temperature range 50-2000 K is 0.009%, the maximum is about 0.02%.
Random errors of calculated values are calculated with a confidence probability of 0.997 by the formula
Where is the average value of the thermodynamic function; - the value of the same function, obtained by the th equation from the system containing equations.
Tables 1-4 show the values of the thermodynamic functions of air, and tables 5-8 show the corresponding random errors. The error values in Tables 5-8 are presented for a part of the isobars, and the values for the intermediate isobars can be obtained with acceptable accuracy by linear interpolation. Random errors in the calculated values reflect the scatter of the latter relative to the averaged equation of state; for density, they are significantly less than the root-mean-square error of describing the initial array of experimental data, which serves as an integral estimate and includes large deviations for some data characterized by scatter.
Table 1
Air density
Continuation
kg/m, at , MPa, |
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table 2
Air enthalpy
Continuation
KJ/kg, at , MPa, |
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Table 3
Air entropy
Continuation
KJ/(kg, K), at , MPa, |
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Table 4
Isobaric heat capacity of air
________________
* The text of the document corresponds to the original. - Database manufacturer's note.
Continuation
KJ/(kg, K), at , MPa, |
|||||||||||
Table 5. Root-mean-square random errors of calculated density values
, %, at , MPa |
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Table 6. Root-mean-square random errors of calculated enthalpy values
KJ/kg, at , MPa |
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In connection with the use of the virial form, the equations of state of the tables do not claim to be exact description thermodynamic properties in the vicinity of the critical point (126-139 K, 190-440 kg/m).
Information about experimental studies of the thermodynamic properties of air, the methodology for compiling the equation of state and calculating tables, the consistency of calculated values with experimental data, as well as more detailed tables containing additional information about isochoric heat capacity, sound speed, heat of evaporation, choke effect, some derivatives and about properties on the boiling and condensation curves are given in .
BIBLIOGRAPHY
1. Hlborn L., Schultre H. die Druckwage und die Isothermen von Luft, Argon und Helium Zwischen 0 und 200 °C. - Ann. Phys. 1915 m, Bd 47, N 16, S.1089-1111.
2. Michels A., Wassenaar T., Van Seventer W. Isotherms of air between 0 °C and 75 °C and at pressures up to 2200 atm. -Appl. sci. Res., 1953, vol. 4, No. 1, p.52-56.
3. Compressibility isotherms of air at temperatures between -25 °C and -155 °C and at densities up to 560 Amagats (Pressures up to 1000 atmospheres) / Michels A.. Wassenaar T., Levelt J.M., De Graaff W. - Appl . sci. Res., 1954, vol. A 4, N 5-6, p.381-392.
4. Experimental study of specific volumes of air / Vukalovich M.P., Zubarev V.N., Aleksandrov A.A., Kozlov A.D. - Thermal power engineering, 1968, N 1, pp. 70-73.
5. Romberg H. Neue Messungen der thermischen ler Luft bei tiefen Temperaturen and die Berechnung der kalorischen mit Hilfe des Kihara-Potentials. - VDl-Vorschungsheft, 1971, - N 543, S.1-35.
6. Blance W. Messung der thermischen von Luft im Zweiphasengebiet und Seiner Umgebung. Dissertation zur Erlangung des Grades eines Doctor-Ingenieurs/. Bohum., 1973.
7. Measurement of air density at temperatures of 78-190 K up to a pressure of 600 bar / Vasserman A.A., Golovsky E.A., Mitsevich E.P., Tsymarny V.A., M., 1975. (Dep. in VINITI 28.07 .76 N 2953-76).
8. H. Landolt, R. Zahlenwerte und Funktionen aus Physik, Chemie, Astronomic, Geophysik und Technik. Berlin., Springer Verlag, 1961, Bd.2.
9. Tables of thermal properties of gases. Wachington., Gov. print, off., 1955, XI. (U.S. Dep. of commerce. NBS. Girc. 564).
10. Thermodynamic properties of air / Sychev V.V., Wasserman A.A., Kozlov A.D. and others. M., Publishing house of standards, 1978.
Lab #1
Definition of mass isobaric
air heat capacity
Heat capacity is the heat that must be supplied to a unit amount of a substance in order to heat it by 1 K. A unit amount of a substance can be measured in kilograms, cubic meters under normal physical conditions and kilomoles. A kilomole of a gas is the mass of a gas in kilograms, numerically equal to its molecular weight. Thus, there are three types of heat capacities: mass c, J/(kg⋅K); volume c', J/(m3⋅K) and molar, J/(kmol⋅K). Since a kilomole of gas has a mass μ times greater than one kilogram, a separate designation for the molar heat capacity is not introduced. Relations between heat capacities:
where = 22.4 m3/kmol is the volume of a kilomole of an ideal gas under normal physical conditions; is the density of the gas under normal physical conditions, kg/m3.
The true heat capacity of a gas is the derivative of heat with respect to temperature:
The heat supplied to the gas depends on the thermodynamic process. It can be determined from the first law of thermodynamics for isochoric and isobaric processes:
Here, is the heat supplied to 1 kg of gas in the isobaric process; is the change in the internal energy of the gas; is the work of gases against external forces.
In essence, formula (4) formulates the 1st law of thermodynamics, from which the Mayer equation follows:
If we put = 1 K, then, that is, the physical meaning of the gas constant is the work of 1 kg of gas in an isobaric process when its temperature changes by 1 K.
Mayer's equation for 1 kilomole of gas is
where = 8314 J/(kmol⋅K) is the universal gas constant.
In addition to the Mayer equation, the isobaric and isochoric mass heat capacities of gases are interconnected through the adiabatic index k (Table 1):
Table 1.1
Values of adiabatic exponents for ideal gases
Atomicity of gases | |
Monatomic gases | |
Diatomic gases | |
Tri- and polyatomic gases |
GOAL OF THE WORK
Anchoring theoretical knowledge according to the basic laws of thermodynamics. Practical development of the method for determining the heat capacity of air based on the energy balance.
Experimental determination of the specific mass heat capacity of air and comparison of the obtained result with a reference value.
1.1. Description of the laboratory setup
The installation (Fig. 1.1) consists of a brass pipe 1 with an inner diameter d =
= 0.022 m, at the end of which there is an electric heater with thermal insulation 10. An air flow moves inside the pipe, which is supplied 3. The air flow can be controlled by changing the fan speed. In pipe 1, a tube of full pressure 4 and excess static pressure 5 are installed, which are connected to pressure gauges 6 and 7. In addition, a thermocouple 8 is installed in pipe 1, which can move along the cross section simultaneously with the full pressure tube. The EMF value of the thermocouple is determined by potentiometer 9. The heating of air moving through the pipe is controlled using a laboratory autotransformer 12 by changing the heater power, which is determined by the readings of the ammeter 14 and voltmeter 13. The air temperature at the outlet of the heater is determined by thermometer 15.
1.2. EXPERIMENTAL TECHNIQUE
Heat flow of the heater, W:
where I is current, A; U – voltage, V; = 0.96; =
= 0.94 - heat loss coefficient.
Fig.1.1. Scheme of the experimental setup:
1 - pipe; 2 - confuser; 3 – fan; 4 - tube for measuring dynamic pressure;
5 - branch pipe; 6, 7 – differential pressure gauges; 8 - thermocouple; 9 - potentiometer; 10 - insulation;
11 - electric heater; 12 – laboratory autotransformer; 13 - voltmeter;
14 - ammeter; 15 - thermometer
Heat flux perceived by air, W:
where m is the mass air flow, kg/s; – experimental, mass isobaric heat capacity of air, J/(kg K); – air temperature at the exit from the heating section and at the entrance to it, °C.
Mass air flow, kg/s:
. (1.10)
Here, is the average air velocity in the pipe, m/s; d is the inner diameter of the pipe, m; - air density at temperature , which is found by the formula, kg/m3:
, (1.11)
where = 1.293 kg/m3 is the air density under normal physical conditions; B – pressure, mm. rt. st; - excess static air pressure in the pipe, mm. water. Art.
Air velocities are determined by dynamic head in four equal sections, m/s:
where is the dynamic head, mm. water. Art. (kgf/m2); g = 9.81 m/s2 is the free fall acceleration.
Average air velocity in the pipe section, m/s:
The average isobaric mass heat capacity of air is determined from formula (1.9), into which the heat flux is substituted from equation (1.8). The exact value of the heat capacity of air at an average air temperature is found according to the table of average heat capacities or according to the empirical formula, J / (kg⋅K):
. (1.14)
Relative error of experiment, %:
. (1.15)
1.3. Conducting the experiment and processing
measurement results
The experiment is carried out in the following sequence.
1. The laboratory stand is turned on and after the stationary mode is established, the following readings are taken:
Dynamic air pressure at four points of equal sections of the pipe;
Excessive static air pressure in the pipe;
Current I, A and voltage U, V;
Inlet air temperature, °С (thermocouple 8);
Outlet temperature, °С (thermometer 15);
Barometric pressure B, mm. rt. Art.
The experiment is repeated for the next mode. The measurement results are entered in Table 1.2. Calculations are performed in table. 1.3.
Table 1.2
Measurement table
Value name | |||
Air inlet temperature, °C | |||
Outlet air temperature, °C |
|||
Dynamic air pressure, mm. water. Art. | |||
Excessive static air pressure, mm. water. Art. |
|||
Barometric pressure B, mm. rt. Art. |
|||
Voltage U, V |
Table 1.3
Calculation table
Name of quantities |
|
|||
Dynamic head, N/m2 | ||||
Average inlet flow temperature, °C |
The main physical properties of air are considered: air density, its dynamic and kinematic viscosity, specific heat capacity, thermal conductivity, thermal diffusivity, Prandtl number and entropy. The properties of air are given in tables depending on the temperature at normal atmospheric pressure.
Air density versus temperature
A detailed table of dry air density values at various temperatures and normal atmospheric pressure is presented. What is the density of air? The density of air can be analytically determined by dividing its mass by the volume it occupies. under given conditions (pressure, temperature and humidity). It is also possible to calculate its density using the ideal gas equation of state formula. To do this, you need to know the absolute pressure and temperature of the air, as well as its gas constant and molar volume. This equation allows you to calculate the density of air in a dry state.
On practice, to find out what is the density of air at different temperatures, it is convenient to use ready-made tables. For example, the given table of atmospheric air density values depending on its temperature. The air density in the table is expressed in kilograms per cubic meter and is given in the temperature range from minus 50 to 1200 degrees Celsius at normal atmospheric pressure (101325 Pa).
| t, °С | ρ, kg / m 3 | t, °С | ρ, kg / m 3 | t, °С | ρ, kg / m 3 | t, °С | ρ, kg / m 3 |
|---|---|---|---|---|---|---|---|
| -50 | 1,584 | 20 | 1,205 | 150 | 0,835 | 600 | 0,404 |
| -45 | 1,549 | 30 | 1,165 | 160 | 0,815 | 650 | 0,383 |
| -40 | 1,515 | 40 | 1,128 | 170 | 0,797 | 700 | 0,362 |
| -35 | 1,484 | 50 | 1,093 | 180 | 0,779 | 750 | 0,346 |
| -30 | 1,453 | 60 | 1,06 | 190 | 0,763 | 800 | 0,329 |
| -25 | 1,424 | 70 | 1,029 | 200 | 0,746 | 850 | 0,315 |
| -20 | 1,395 | 80 | 1 | 250 | 0,674 | 900 | 0,301 |
| -15 | 1,369 | 90 | 0,972 | 300 | 0,615 | 950 | 0,289 |
| -10 | 1,342 | 100 | 0,946 | 350 | 0,566 | 1000 | 0,277 |
| -5 | 1,318 | 110 | 0,922 | 400 | 0,524 | 1050 | 0,267 |
| 0 | 1,293 | 120 | 0,898 | 450 | 0,49 | 1100 | 0,257 |
| 10 | 1,247 | 130 | 0,876 | 500 | 0,456 | 1150 | 0,248 |
| 15 | 1,226 | 140 | 0,854 | 550 | 0,43 | 1200 | 0,239 |
At 25°C, air has a density of 1.185 kg/m 3 . When heated, the density of air decreases - the air expands (its specific volume increases). With an increase in temperature, for example, up to 1200 ° C, a very low air density is achieved, equal to 0.239 kg / m 3, which is 5 times less than its value at room temperature. In general, the decrease in heating allows a process such as natural convection to take place and is used, for example, in aeronautics.
If we compare the density of air with respect to, then air is lighter by three orders of magnitude - at a temperature of 4 ° C, the density of water is 1000 kg / m 3, and the density of air is 1.27 kg / m 3. It is also necessary to note the value of air density under normal conditions. Normal conditions for gases are those under which their temperature is 0 ° C, and the pressure is equal to normal atmospheric pressure. Thus, according to the table, air density under normal conditions (at NU) is 1.293 kg / m 3.
Dynamic and kinematic viscosity of air at different temperatures
When performing thermal calculations, it is necessary to know the value of air viscosity (viscosity coefficient) at different temperatures. This value is required to calculate the Reynolds, Grashof, Rayleigh numbers, the values of which determine the flow regime of this gas. The table shows the values of the coefficients of dynamic μ and kinematic ν air viscosity in the temperature range from -50 to 1200°C at atmospheric pressure.
The viscosity of air increases significantly with increasing temperature. For example, the kinematic viscosity of air is 15.06 10 -6 m 2 / s at a temperature of 20 ° C, and with an increase in temperature to 1200 ° C, the viscosity of the air becomes equal to 233.7 10 -6 m 2 / s, that is, it increases 15.5 times! The dynamic viscosity of air at a temperature of 20°C is 18.1·10 -6 Pa·s.
When air is heated, the values of both kinematic and dynamic viscosity increase. These two quantities are interconnected through the value of air density, the value of which decreases when this gas is heated. An increase in the kinematic and dynamic viscosity of air (as well as other gases) during heating is associated with a more intense vibration of air molecules around their equilibrium state (according to the MKT).
| t, °С | μ 10 6 , Pa s | ν 10 6, m 2 / s | t, °С | μ 10 6 , Pa s | ν 10 6, m 2 / s | t, °С | μ 10 6 , Pa s | ν 10 6, m 2 / s |
|---|---|---|---|---|---|---|---|---|
| -50 | 14,6 | 9,23 | 70 | 20,6 | 20,02 | 350 | 31,4 | 55,46 |
| -45 | 14,9 | 9,64 | 80 | 21,1 | 21,09 | 400 | 33 | 63,09 |
| -40 | 15,2 | 10,04 | 90 | 21,5 | 22,1 | 450 | 34,6 | 69,28 |
| -35 | 15,5 | 10,42 | 100 | 21,9 | 23,13 | 500 | 36,2 | 79,38 |
| -30 | 15,7 | 10,8 | 110 | 22,4 | 24,3 | 550 | 37,7 | 88,14 |
| -25 | 16 | 11,21 | 120 | 22,8 | 25,45 | 600 | 39,1 | 96,89 |
| -20 | 16,2 | 11,61 | 130 | 23,3 | 26,63 | 650 | 40,5 | 106,15 |
| -15 | 16,5 | 12,02 | 140 | 23,7 | 27,8 | 700 | 41,8 | 115,4 |
| -10 | 16,7 | 12,43 | 150 | 24,1 | 28,95 | 750 | 43,1 | 125,1 |
| -5 | 17 | 12,86 | 160 | 24,5 | 30,09 | 800 | 44,3 | 134,8 |
| 0 | 17,2 | 13,28 | 170 | 24,9 | 31,29 | 850 | 45,5 | 145 |
| 10 | 17,6 | 14,16 | 180 | 25,3 | 32,49 | 900 | 46,7 | 155,1 |
| 15 | 17,9 | 14,61 | 190 | 25,7 | 33,67 | 950 | 47,9 | 166,1 |
| 20 | 18,1 | 15,06 | 200 | 26 | 34,85 | 1000 | 49 | 177,1 |
| 30 | 18,6 | 16 | 225 | 26,7 | 37,73 | 1050 | 50,1 | 188,2 |
| 40 | 19,1 | 16,96 | 250 | 27,4 | 40,61 | 1100 | 51,2 | 199,3 |
| 50 | 19,6 | 17,95 | 300 | 29,7 | 48,33 | 1150 | 52,4 | 216,5 |
| 60 | 20,1 | 18,97 | 325 | 30,6 | 51,9 | 1200 | 53,5 | 233,7 |
Note: Be careful! The viscosity of air is given to the power of 10 6 .
Specific heat capacity of air at temperatures from -50 to 1200°С
A table of the specific heat capacity of air at various temperatures is presented. The heat capacity in the table is given at constant pressure (isobaric heat capacity of air) in the temperature range from minus 50 to 1200°C for dry air. What is the specific heat capacity of air? The value of specific heat capacity determines the amount of heat that must be supplied to one kilogram of air at constant pressure to increase its temperature by 1 degree. For example, at 20°C, to heat 1 kg of this gas by 1°C in an isobaric process, 1005 J of heat is required.
The specific heat capacity of air increases as its temperature rises. However, the dependence of the mass heat capacity of air on temperature is not linear. In the range from -50 to 120°C, its value practically does not change - under these conditions, the average heat capacity of air is 1010 J/(kg deg). According to the table, it can be seen that the temperature begins to have a significant effect from a value of 130°C. However, air temperature affects its specific heat capacity much weaker than its viscosity. So, when heated from 0 to 1200°C, the heat capacity of air increases only 1.2 times - from 1005 to 1210 J/(kg deg).
It should be noted that the heat capacity of moist air is higher than that of dry air. If we compare air, it is obvious that water has a higher value and the water content in the air leads to an increase in specific heat.
| t, °С | C p , J/(kg deg) | t, °С | C p , J/(kg deg) | t, °С | C p , J/(kg deg) | t, °С | C p , J/(kg deg) |
|---|---|---|---|---|---|---|---|
| -50 | 1013 | 20 | 1005 | 150 | 1015 | 600 | 1114 |
| -45 | 1013 | 30 | 1005 | 160 | 1017 | 650 | 1125 |
| -40 | 1013 | 40 | 1005 | 170 | 1020 | 700 | 1135 |
| -35 | 1013 | 50 | 1005 | 180 | 1022 | 750 | 1146 |
| -30 | 1013 | 60 | 1005 | 190 | 1024 | 800 | 1156 |
| -25 | 1011 | 70 | 1009 | 200 | 1026 | 850 | 1164 |
| -20 | 1009 | 80 | 1009 | 250 | 1037 | 900 | 1172 |
| -15 | 1009 | 90 | 1009 | 300 | 1047 | 950 | 1179 |
| -10 | 1009 | 100 | 1009 | 350 | 1058 | 1000 | 1185 |
| -5 | 1007 | 110 | 1009 | 400 | 1068 | 1050 | 1191 |
| 0 | 1005 | 120 | 1009 | 450 | 1081 | 1100 | 1197 |
| 10 | 1005 | 130 | 1011 | 500 | 1093 | 1150 | 1204 |
| 15 | 1005 | 140 | 1013 | 550 | 1104 | 1200 | 1210 |
Thermal conductivity, thermal diffusivity, Prandtl number of air
The table shows such physical properties of atmospheric air as thermal conductivity, thermal diffusivity and its Prandtl number depending on temperature. The thermophysical properties of air are given in the range from -50 to 1200°C for dry air. According to the table, it can be seen that the indicated properties of air depend significantly on temperature and the temperature dependence of the considered properties of this gas is different.
Which is necessary to change the temperature of the working fluid, in this case, air, by one degree. The heat capacity of air is directly dependent on temperature and pressure. However, for research different types heat capacities can be used various methods.
Mathematically, the heat capacity of air is expressed as the ratio of the amount of heat to the increment in its temperature. The heat capacity of a body having a mass of 1 kg is called the specific heat. The molar heat capacity of air is the heat capacity of one mole of a substance. The heat capacity is indicated - J / K. Molar heat capacity, respectively, J / (mol * K).
Heat capacity can be considered a physical characteristic of a substance, in this case air, if the measurement is carried out in constant conditions. Most often, such measurements are carried out at constant pressure. This is how the isobaric heat capacity of air is determined. It increases with increasing temperature and pressure, and is also a linear function of these quantities. In this case, the temperature change occurs at a constant pressure. To calculate the isobaric heat capacity, it is necessary to determine the pseudocritical temperature and pressure. It is determined using reference data.
Heat capacity of air. Peculiarities
Air is a gas mixture. When considering them in thermodynamics, the following assumptions were made. Each gas in the mixture must be evenly distributed throughout the volume. Thus, the volume of the gas is equal to the volume of the entire mixture. Each gas in the mixture has its own partial pressure, which it exerts on the walls of the vessel. Each of the components gas mixture should have a temperature equal to the temperature of the entire mixture. In this case, the sum of the partial pressures of all components is equal to the pressure of the mixture. Calculation of the heat capacity of air is performed on the basis of data on the composition of the gas mixture and the heat capacity of individual components.
Heat capacity ambiguously characterizes a substance. From the first law of thermodynamics, we can conclude that the internal energy of a body varies not only depending on the amount of heat received, but also on the work done by the body. At various conditions the course of the heat transfer process, the work of the body may vary. Thus, the same amount of heat communicated to the body can cause changes in temperature and internal energy of the body that are different in value. This feature is characteristic only for gaseous substances. Unlike hard and liquid bodies, gaseous substances, can greatly change volume and do work. That is why the heat capacity of air determines the nature of the thermodynamic process itself.
However, at a constant volume, the air does not do work. Therefore, the change in internal energy is proportional to the change in its temperature. The ratio of the heat capacity in a constant pressure process to the heat capacity in a constant volume process is part of the adiabatic process formula. It is denoted by the Greek letter gamma.
From the history
The terms "heat capacity" and "amount of heat" do not describe their essence very well. This is due to the fact that they came to modern science from the theory of caloric, which was popular in the eighteenth century. The followers of this theory considered heat as a kind of imponderable substance contained in bodies. This substance can neither be destroyed nor created. The cooling and heating of bodies was explained by a decrease or increase in the caloric content, respectively. Over time, this theory was recognized as untenable. She could not explain why the same change in the internal energy of a body is obtained when transferring different amounts of heat to it, and also depends on the work done by the body.