Thermal contacts

Between two systems there can be a variety of interactions. Thennodynamic equilibrium of a system implies themial, chemical and mechanical equilibria. It is therefore logical to consider, in sequence, the following interactions between two systems thermal contact, which enables the two systems to share energy material contact, which enables exchange of particles between them and pressure transmitting contact, which allows an exchange of volume between the two systems. In each of the cases, the combined composite system is... 

The distillation heads Fig. 22(F) and Fig. 22(G) can be fitted with thermometers having a ground-glass cone just above the bulb (Fig. 22(M)). These are expensive, and it is usually more convenient to fit a thermometer pocket (Fig. 22(N)) which consists of a small well , fitting as shown into the neck of the flask. A small volume of mercury is placed in the well just to cover the bulb of a conventional thermometer, and thus provides excellent thermal contact between the thermometer and the sides of the pocket. 

Electrical heating is accomplished with resistance bauds or ribbons which must be electrically insulated from the machine body but in good thermal contact with it. The heaters must be carefully spaced to avoid a succession of hot and cold areas. Sometimes they are mounted in aluminum blocks shaped to conform to the container walls. Their effective temperature range is 150 to 500°C (about 300 to 930°F). Temperature control is precise, maintenance and supervision costs are low, and conversion of electrical energy to useful heat is almost 100 percent. The cost of electrical energy is usually large, however, and may be prohibitive. 

Consider now, as an illustration, a confined fluid in material and thermal contact with a bulk reservoir and under fixed normal stress For simplicity we assume the substrates to be in fixed registry a. = ay = 0 and the... 

Gardner, K A. and T. C. Carnavas, Thermal-Contact Resistance to Pinned Tubing, Trans, of ASMEJour, of Heat Transfer, Paper No. 59-A-135. 

The purpose of the evaporator is to receive low-pressure, low-temperature fluid from the expansion valve and to bring it in close thermal contact with the load. The refrigerant takes up its latent heat from the load and leaves the evaporator as a drygas. Evaporators are classified according to their refrigerant flow pattern and their function. 

Wells should slope downwards into the pipe, so that they can be part filled with liquid to provide better thermal contact. Where a pipe temperature is a critical factor in the operation of a system, it is usually worth fitting a permanent thermometer. 

When a chemical reaction takes place, we consider the substances involved, reactants and products, to be the system. The surroundings include the vessel in which the reaction takes place (test tube, beaker, and so on) and the air or other material in thermal contact with the reaction system. 

So let us measure the temperature of a sample of gas A by placing it in thermal contact with a sample of gas B (our thermometer). There will be heat flow between the two gas samples if they are initially at different temperatures. Energy is transferred from the hotter gas to the cooler gas. When heat flow ceases, the gases have reached thermal equilibrium. Then the gases have the same temperature. 

Like pressure, temperature is an intensive variable. In a qualitative sense it may be thought of as the potential that drives the flow of heat. This can be seen by referring to Figure l.l. If two systems are in thermal contact, one at temperature 7) and the other at temperature 73, then heat will be exchanged between the two systems so that q flows from system 1 to system 2 and q2 flows from system 2 to system 1. If 73 > 7), then the rate of flow of heat from system 2 to system 1 will be greater than the rate of flow of heat from system 1 to system 2. The net effect will be that system 1 will increase in temperature and system 2 will decrease in temperature. With time, the difference between the two heat flow rates decreases until it becomes zero. When this occurs, 73 = 73 and the two systems are said to be in thermal equilibrium the flow of heat from system 1 to 2 balances the flow of heat from system 2 to 1. 

In the parallel-plate method, the heat flux downward is measured hy a fluxmeter under which the thermal bond material and the molten salt of interest are located. Because the thermal transfer is only conductive if the thermal contacts are perfect, the balance of heat flow through the... 

Now place the subsystem in thermal contact with the two reservoirs discussed at the start of this section. In this case the energy moment can change by the internal processes just discussed, or by exchange with the reservoirs,... 

The determination of these curves requires not only the measurement of small amounts of heat in a microcalorimeter, but also the simultaneous determination of the corresponding quantity of adsorbed gas. Volumetric measurements are to be preferred to gravimetric measurements for these determinations because it would be very difficult indeed to ensure a good, and reproducible, thermal contact between a sample of adsorbent, hanging from a balance beam, and the inner cell of a heat-flow calorimeter. 

The adsorption cell (C in Fig. 15) which contains the adsorbent must be placed in the inner cell of the calorimeter and a good thermal contact must be established between the sample and the sensing elements of the calorimeter. The mechanical contact between the volumetric line and the calorimeter occurs, therefore, in the calorimeter cell itself. Thence, any relative movement or vibrations between these parts of the apparatus must be strictly avoided. This necessitates the very careful installation of the whole apparatus, especially if experiments of long duration are to be made. 

In these gages, a wire inside the gas whose pressure is to be measured, is electrically heated by a constant power (see Fig. 1.29). As the gas density decreases, the heat loss from the filament to the envelope walls decreases and hence the filament temperature increases (not linearly). The temperature (200-300°C) is read by a thermocouple in thermal contact with the wire. 

Other types of thermal switches have been built, e.g. mechanical switches which can be turned on and off easily, but which produce heat in the switching and are therefore used at T > IK. Thermal contact is made by pressing together metallic surfaces. Conductance of a few mW/K can be obtained in the on state [14,30,41-45]. Another disadvantage of these switches are the large forces (typically 10 kg) needed to produce a good contact. Nowadays, the use of these switches is not common. 

One of the main problems encountered in low-temperature experiments is the realization of good thermal contacts between the parts of a cryogenic system. 

As can be noted in Fig. 4.4, even if for 0.01 K < T < 0.2K, experimental data confirm the theory of acoustic mismatch with A - Rk T3 10 2 [m2K4/W] the good thermal contact at higher temperatures is not yet explained, in spite of the studies of the problem [57,61-68], Referring to Fig. 4.4, data of Rc between solids are only indicative of the temperature dependence, since the pressure on the contact is not known. 

In 1959, Little [69] extended the acoustic mismatch model to interfaces between solids. The experiments have revealed that in this case, the thermal contact resistance between solids is higher than that evaluated from the model and that data are less reproducible. 

In 1987, Swartz [73] measured the thermal boundary resistance between metal films and the dielectric substrates onto which the films were deposited, in the range 0.6-200 K. A typical example is the measurement of the thermal contact resistance between indium and sapphire [72]. To minimize the dependence on surface irregularities, indium was vacuum deposited onto the sapphire rods the two surfaces were then pressed together and annealed. Analogous measurements have been carried out also with lead and aluminium. In all these cases, it has been clear that the contact resistance was strongly dependent on the sample preparation. In particular, obtained data suggest that the contact between the two materials was not complete. 

Fig. 4.4. Thermal contact resistance Rk multiplied by A T3 between liquid helium and various solids and also...

---