Irreversible Thermodynamics of the Thermal Characteristics of Porous Insulators

1962 ◽  
Vol 29 (2) ◽  
pp. 425-428 ◽  
Author(s):  
R. G. Mokadam
Keyword(s):  
Thermal Conductivity ◽  
Heat Flow ◽  
Temperature Gradient ◽  
Pressure Gradient ◽  
Convective Heat ◽  
Frictional Force ◽  
Thermal Gradient ◽  
Gas Flow ◽  
Irreversible Processes ◽  
Set Up

The entropy equation contains terms which indicate entropy generation due to two irreversible processes: Heat flow in the presence of temperature gradient, and gas flow in the presence of frictional force. These flows are assumed to be linearly dependent upon the temperature gradient and the frictional force. This assumption includes two cross phenomena: Convective heat transfer (set up by pressure gradient), and free convection (set up by temperature gradient). They are interdependent. Usually the frictional force is equal to the gaseous-phase pressure gradient. When this pressure gradient is zero, the heat flow depends only upon the thermal gradient. By entrapping the gas in the porous medium, the gas flow is stopped. This gives rise to a pressure gradient which sets up a convective heat flow opposing that due to thermal gradient. Consequently, the thermal conductivity of the porous insulator decreases. Experimental work on Tritex insulation material indicated a 22 per cent decrease in thermal conductivity.

Geothermal Data from the Granduc Area, Northern Coast Mountains of British Columbia

1972 ◽  
Vol 9 (10) ◽  
pp. 1333-1337 ◽  
Author(s):  
W. H. Mathews
Keyword(s):  
Thermal Conductivity ◽  
British Columbia ◽  
Heat Flow ◽  
Thermal Gradient ◽  
Temperature Measurements ◽  
Simplified Model ◽  
Northern Coast ◽  
Rock Glacier ◽  
Coast Mountains

Temperature measurements have been obtained from 80 points along the Granduc haulage tunnel, at depths of as much as 1.5 km below the surface. These fit, within 1 °C, a simplified model assuming, among other things, uniform thermal conductivity of the rocks and a temperature at rock–glacier contacts of 0 °C. For these assumptions a generalized thermal gradient (with effects of topographic irregularity removed) is about 26 mK m−1 (26 °C/km). With the thermal conductivity of a suite of rocks from the tunnel averaging 2.72 ± 12 W m−1K−1 (6.50 ±.28 cal/cm s °C) present heat flow of about 73 mW m−2 (1.74 μcal/cm2 s) can be derived.

Thermal Transpiration Based Propulsion

2014 ◽  
Author(s):  
Ryan Falkenstein-Smith ◽  
Pingying Zeng ◽  
Tyler Culp ◽  
Jeongmin Ahn
Keyword(s):  
Temperature Gradient ◽  
Pressure Gradient ◽  
Gas Flow ◽  
Average Pore Size ◽  
Combustion Process ◽  
Gas Diffusion ◽  
Electrical Heating ◽  
Average Pore ◽  
Porous Substance ◽  
Thermal Transpiration

Thermal transpiration based propulsion is studied. Thermal transpiration describes flowing of the gas through a narrow channel with an imposed temperature gradient. As gas flows from the cold to hot side in the chamber, a pressure gradient is created across the channel induced by the temperature gradient. Between the two sides of the chamber an aerogel substance, which functions as an excellent insulator, is used as a thermal transpiration membrane and allows gas diffuse to the hot chamber. The induced pressure gradient is the driving factor in the propulsion of air, or any gas, into the chamber and through the porous membrane. The use of a porous substance such as aerogel as the transpiration membrane and a pressure gradient served as the two requirements in order to successfully achieve thermal transpiration. The gas diffusion through the aerogel transpiration membrane indicates that the average pore size of the aerogel must be comparable with the free path of the molecules. This concept can be taken further if the outlet chamber served as a combustion reactor. The flowing gas is motivated by the heat produced from the combustion process. Along with the exceptionally low thermal conductivity of the aerogel, the gas flow permits the propulsion device to be self-sustaining. The implications of providing a self-sustaining heat source signify that no external electrical heating is required. The effectiveness of this device can be measured as a function of the porous size of the membrane and the temperature difference applied to the system and pressure gradient created.

scholarly journals The influence of pressure on the thermal conductivity of liquid He ll

1939 ◽  
Vol 171 (945) ◽  
pp. 242-250 ◽  
Keyword(s):  
Heat Transfer ◽  
Thermal Conductivity ◽  
Mass Transfer ◽  
Heat Flow ◽  
Low Temperature ◽  
Temperature Gradient ◽  
Standard Method ◽  
The Other ◽  
Surface Films ◽  
Very High

The unusual characteristics of heat transfer in liquid He II have been reported in several recent papers. The very high thermal conductivity of the low-temperature modification of liquid helium was first noted by Keesom and Keesom (1935). It was then found by Allen, Peierls and Uddin (1937) and subsequently verified by Keesom, Keesom and Saris (1938) that the rate of transfer of heat varied with the temperature gradient. The discovery of the momentum transfer accompanying heat flow in He II which was made by Allen and Jones (1938) and the work on mobile surface films of the liquid done by Daunt and Mendelssohn (1938) show that a large part of the heat must be carried by some form of mass transfer. Several ideas and theories to explain the phenomena have been put forward by Kapitza (1938), Jones (1938), Michels, Bijl and de Boer (1938), Tisza (1938) and Keesom and Taconis (1938). The experimental evidence is as yet too meagre to prove or disprove any of the theories. It was with the intention of adding to the data already known concerning the properties of liquid He II that the present research was undertaken. The apparatus which was used is shown in fig. 1. The thermal conductivity was measured by a standard method. A constant supply of heat was supplied to one end of a long capillary containing liquid He II, and the other end was maintained at the constant temperature of the He II bath. Temperatures were observed at two points along the capillary.

Heat transfer in liquid helium below 1°K

1955 ◽  
Vol 231 (1187) ◽  
pp. 545-555 ◽  
Keyword(s):  
Heat Transfer ◽  
Thermal Conductivity ◽  
Heat Flow ◽  
Temperature Gradient ◽  
Thermal Resistance ◽  
Specific Heat ◽  
Liquid Helium ◽  
Practical Importance ◽  
Heat Pulses

The thermal conductivity of liquid helium has been measured between 0.2 and 1.0° K. Below 0.6° K the heat flow is exactly proportional to the temperature gradient and the thermal conductivity is proportional to the specific heat and the diameter of the specimen. Thus the sole mechanism of heat transfer appears to be by phonons which are scattered only at the boundaries of the specimen. These results are in satisfactory accord with previous theoretical discussions and with measurements of the propagation of heat pulses in the liquid. The experiment also afforded the opportunity of making subsidiary measurements of the thermal resistance of the boundary between a metal and liquid helium. Besides being of practical importance, the results show that some modification is called for in the existing theoretical treatments.

scholarly journals Development of Experimental Setup for Measuring Thermal Conductivity Characteristics of Soil

2018 ◽  
Vol 37 (4) ◽  
pp. 559-568 ◽  
Author(s):  
Gul Muhammad ◽  
Amanullah Marri ◽  
Abdul Majeed Shar
Keyword(s):  
Thermal Conductivity ◽  
Heat Flow ◽  
Thermal Stresses ◽  
Measurement Techniques ◽  
Conductivity Measurement ◽  
Theoretical Background ◽  
Experimental Setup ◽  
Engineering Structures ◽  
Steady State Heat ◽  
Set Up

Thermal conductivity displays a key role in design of engineering structures where, thermal stresses resulting from heat and temperatures are of concern. Significant efforts were made to measure the thermal conductivity of different materials. For thermal conductivity characterization of soil samples it is essential to have very flexible set-up. Hence, this paper provides details about indigenously developed experimental setup for thermal conductivity measurement. The design of this newly developed setup is based on the basic principle of steady state heat flow. This experimental setup is designed in order to measure the thermal conductivity of various materials such as soils, rocks, concrete and any type of unbonded and bonded materials. In this paper, initially the theoretical background of the measurement techniques and the principle of heat flow are described, followed by design description and working procedure. The design has been kept very simple, adjustable for varying type and size of specimens and easy to operate with excellent level of accuracy as evident from system calibration. The accuracy and precision of the newly developed setup was verified by testing reference materials of known thermal conductivity and in the test results a high correlation coefficient (R^2 = 0.999) between experimental data and fitting curve was achieved.

scholarly journals Development of Experimental Setup for Measuring Thermal Conductivity Characteristics of Soil

2018 ◽  
Vol 37 (4) ◽  
pp. 559-568
Author(s):  
Gul Muhammad ◽  
Amanullah Marri ◽  
Abdul Majeed Shar
Keyword(s):  
Thermal Conductivity ◽  
Heat Flow ◽  
Thermal Stresses ◽  
Measurement Techniques ◽  
Conductivity Measurement ◽  
Theoretical Background ◽  
Experimental Setup ◽  
Engineering Structures ◽  
Steady State Heat ◽  
Set Up

Thermal conductivity displays a key role in design of engineering structures where, thermal stresses resulting from heat and temperatures are of concern. Significant efforts were made to measure the thermal conductivity of different materials. For thermal conductivity characterization of soil samples it is essential to have very flexible set-up. Hence, this paper provides details about indigenously developed experimental setup for thermal conductivity measurement. The design of this newly developed setup is based on the basic principle of steady state heat flow. This experimental setup is designed in order to measure the thermal conductivity of various materials such as soils, rocks, concrete and any type of unbonded and bonded materials. In this paper, initially the theoretical background of the measurement techniques and the principle of heat flow are described, followed by design description and working procedure. The design has been kept very simple, adjustable for varying type and size of specimens and easy to operate with excellent level of accuracy as evident from system calibration. The accuracy and precision of the newly developed setup was verified by testing reference materials of known thermal conductivity and in the test results a high correlation coefficient (R2 = 0.999) between experimental data and fitting curve was achieved.

GEOLOGICAL AND GEOPHYSICAL STUDY FOR ELABORATION OF GEOTHERMAL MODEL IN ORADEA-BAILE FELIX AREA

2020 ◽  
Author(s):  
Laurențiu Asimopolos ◽  
Natalia-Silvia Asimopoli
Keyword(s):  
Thermal Conductivity ◽  
Heat Flow ◽  
Thermal Gradient ◽  
Thermal Inertia ◽  
Geothermal Gradient ◽  
Point Of View ◽  
Hot Water ◽  
Thermal Methods ◽  
Thermal Anomalies ◽  
Average Value

Thermal methods consist of measuring thermal gradient and satellite data, which can be used to determine the Earth's surface temperature and thermal inertia of surficial materials, of thermal infrared radiation emitted at the Earth's surface. Thermal gradient measuring, with a knowledge of the thermal conductivity provides a measure of heat flow. Conditions that may increase or decrease and heat flow are influenced by hydrologic, topographic factors and anomalous thermal conductivity. Also, oxidation of sulphide bodies in-place or on waste deposits, if sufficiently rapid, can generate thermal anomalies, which can provide a measure of the amount of metal being released to the environment. The geothermal gradient on the territory of Romania, the increase of the temperature with the depth, has an average value of 2.5°-3°C/100m, which corresponds to a temperature of 100° C at 3000 m deep. There are many areas where the value of the geothermal gradient differs considerably from this average. For example, in areas where the rock plate suffered rapid dips and the basin was filled with sediment "very young "from a geological point of view, the geothermal gradient may be less than 1° C/100m. On the other hand, in other geothermal areas the gradient exceeds much this average. These areas are true underground thermal reservoirs of potentially high geothermal energy which under certain favourable conditions can be exploited to serve heating installations and domestic hot water systems. The geothermal prospecting for the entire territory of Romania, carried out by temperature measurements allowed the development of geothermal maps, highlighting the temperature distribution at different depths. Geophysical data obtained through various methods and geophysical modelling provide generalized and non-unique solutions to the geometry of underground geological relations as well as to the physical characteristics of different formations. The non-uniqueness of these models (solutions to the direct problem) arises from the impossibility of knowing the boundary conditions between different strata, which together with the propagation equations of the different fields (depending on the geophysical method used for the investigation of the basement) form the systems that offer the solutions of the model

RADIATION-CONVECTIVE HEAT TRANSFER BETWEEN GAS FLOW AND BLUNT BODIES OF POROUS INJECTION OF RADIATION ABSORBING SUBSTANCES

1970 ◽  
Author(s):  
V. P. Motulevich ◽  
M.S. Bespalov ◽  
A.N. Boyko ◽  
V. M. Eroshenko ◽  
E. D. Sergievskii ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Convective Heat Transfer ◽  
Convective Heat ◽  
Gas Flow ◽  
Blunt Bodies ◽  
Porous Injection
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