Heat sink temperature, measurement

The apparatus comprises a rod-shaped heat source surrounded by a tubular sample inside a cylindrical heat sink. The heat sink and the heat source have the same axis. The temperature of the heat sink is controlled, usually by circulating a liquid from a constant temperature bath, and a known power is supplied to the heat source. The temperature difference between the heat source and the heat sink is measured after steady state conditions have been established. The conductivity is obtained from Eq. 8. 

The performance of the cooler was measured at heat sink temperatures of 77°, 156°, and 195°K. Figure 6 shows a plot of AT, the temperature differential between the cooling... 

Temperature measurement is a case in point. A large transducer in close contact with the body whose temperature is being measured will act as a heat sink and consequently produce a localized reduction at the point where the temperature is being measured. On the other hand, if an air gap exists between the transducer and the hot surface then the air (rather than the surface) temperature will be measured. 

In differential thermal analysis, a sample and reference material are placed in the same large metal heat sink. Changes in the heat capacity of the sample are measured by changes in temperature between the sample and the reference materials as they are heated at the same rate. 

In differential scanning calorimetry (DSC), higher precision can be obtained and heat capacities can be measured. The apparatus is similar to that for a DTA analysis, with the primary difference being that the sample and reference are in separate heat sinks that are heated by individual heaters (see the following illustration). The temperatures of the two samples are kept the same by differential heating. Even slight... 

We attempt here to describe the fundamental equations of fluid mechanics and heat transfer. The main emphasis, however, is on understanding the physical principles and on application of the theory to realistic problems. The state of the art in high-heat flux management schemes, pressure and temperature measurement, pressure drop and heat transfer in single-phase and two-phase micro-channels, design and fabrication of micro-channel heat sinks are discussed. 

The experimental investigations of boiling instability in parallel micro-channels have been carried out by simultaneous measurements of temporal variations of pressure drop, fluid and heater temperatures. The channel-to-channel interactions may affect pressure drop between the inlet and the outlet manifold as well as associated temperature of the fluid in the outlet manifold and heater temperature. Figure 6.37 illustrates this phenomenon for pressure drop in the heat sink that contains 13 micro-channels of d = 220 pm at mass flux G = 93.3kg/m s and heat flux q = 200kW/m. The temporal behavior of the pressure drop in the whole boiling system is shown in Fig. 6.37a. The considerable oscillations were caused by the flow pattern alternation, that is, by the liquid/two-phase alternating flow in the micro-channels. The pressure drop FFT is presented in Fig. 6.37b. Under... 

When heat is liberated or absorbed in the calorimeter vessel, a thermal flux is established in the heat conductor and heat flows until the thermal equilibrium of the calorimetric system is restored. The heat capacity of the surrounding medium (heat sink) is supposed to be infinitely large and its temperature is not modified by the amount of heat flowing in or out. The quantity of heat flowing along the heat conductor is evaluated, as a function of time, from the intensity of a physical modification produced in the conductor by the heat flux. Usually, the temperature difference 0 between the ends of the conductor is measured. Since heat is transferred by conduction along the heat conductor, calorimeters of this type are often also named conduction calorimeters (20a). 

Since heat exchange between the calorimeter vessel and the heat sink is not hindered in a heat-flow calorimeter, the temperature changes produced by the thermal phenomenon under investigation are usually very small (less than 10 4 degree in a Calvet microcalorimeter, for instance) (23). For most practical purposes, measurements in a heat-flow calorimeter may be considered as performed under isothermal conditions. 

As we saw in Section 4.4, g s(T) is expected to depend on temperature approximately as T3 (there is always an electrical insulating layer in the contact between the thermistor and the metallic support at the temperature of the heat sink) (Fig. 15.3). For a review of the few existing measurements of electron-phonon decoupling, see ref. [20]. 

An extremely simplified scheme of a calorimeter (composite thermal detector) is shown in Fig. 15.6. The temperature of an absorber A (TA) is measured by a thermometer T. A thermal conductance G forms a thermal link with the heat sink B at the temperature Ts. In the ideal adiabatic situation (G = 0), an absorption of an energy AE produces an absorber temperature increase ... 

As in the case of calorimeters, a bolometer consists of an absorbing element with heat capacity C, which converts the impinging electromagnetic radiation to heat, and which is linked to a heat sink at temperature Ts via a thermal conductance G. The temperature TA of the absorber is measured by a thermometer in thermal contact with the absorber. 

The measurement of an enthalpy change is based either on the law of conservation of energy or on the Newton and Stefan-Boltzmann laws for the rate of heat transfer. In the latter case, the heat flow between a sample and a heat sink maintained at isothermal conditions is measured. Most of these isoperibol heat flux calorimeters are of the twin type with two sample chambers, each surrounded by a thermopile linking it to a constant temperature metal block or another type of heat reservoir. A reaction is initiated in one sample chamber after obtaining a stable stationary state defining the baseline from the thermopiles. The other sample chamber acts as a reference. As the reaction proceeds, the thermopile measures the temperature difference between the sample chamber and the reference cell. The rate of heat flow between the calorimeter and its surroundings is proportional to the temperature difference between the sample and the heat sink and the total heat effect is proportional to the integrated area under the calorimetric peak. A calibration is thus... 

A second improvement in Calvet s calorimeter is that a differential set-up was adopted that aimed to suppress temperature drifts and fluctuations of the heat sink. This was achieved by coupling two calorimetric units in opposition to each other, so the measured thermoelectric force was the difference between the thermoelectric forces of the sample cell and the reference cell. The latter may remain at the temperature of the thermostat while the heat output or input related to the event under investigation occurs in the sample cell. 

The temperature of the block, which works as a heat sink, is controlled very precisely. The heat generated in the system flows to the heat sink and is accurately measured by means of a detector. This is made up of a large number of identical conductive thermocouples (a thermopile) that surround the vessel and connect it to the block in such a way that the vessel and the block temperatures are always close to each other. 

The thermal conductivity (TC) detector consists of four filaments embedded in a stainless-steel or brass block which acts as a heat sink. The TC detector is extremely sensitive to temperature changes and should be insulated to prevent temperature excursions during the time in which it takes to complete an adsorption or desorption measurement. Long-term thermal drift is not significant because of the calibration procedure discussed in the next section and therefore, thermostating is not required. 

The open test method for tempered hybrid systems is the same as that given for vapour pressure systems in A2.4.3 above. However, in addition to measuring the test cell temperature, the rate of pressure rise in the closed containment vessel during tempering should also be measured. The rate of heat release per unit mass, q, can be obtained from measured dT/dt data, suitably corrected for thermal inertia (e.g. by using equation (A2.12)). Equation (A2.4) can be used to determine the rate of permanent gas evolution, QG. As the containment vessel provides a large heat sink, vapour is likely to condense, so that the rate of pressure rise is due only to the non-condensible gas., ... 

---