The Problem:
The heating / cooling rate of a body strongly depends on the environment with which the body itself can exchange energy. It is easy to observe that the cooling rate is higher if a fan is placed near the body. The problem we investigate here is: what happens if we leave the hot body in a vacuum camera? We will present some experiments analysing the cooling processes in three different conditions: in a vacuum camera, in air and in air, but near a fan. This involves to analyse cooling only through irradiation or through irradiation as well as natural and forced convection.The experiments are relatively simple and the needed equipment is readily available. The complex aspect of the experiment is certainly related to the implementation of the vacuum as this requires the use of more sophisticated equipment.
Learning aims:
The aim of this activity is to study the way a body cools in different environmental situations.
A 150 Ω, 10W ceramic resistor.• A K type Thermocouple;• Transducer voltage - temperature module for thermocouple (for example Fluke 80TK already prepared for type K thermocouple);• Multimeter with a resolution of at least 0.1 mV in d.c.;• Electrical cables;• d.c. 40 V power supply • Bell vacuum glass;• Vacuum grease;• Rubber hose vacuum;• Rotary vacuum pump;• Pressure gauge;• PC software for data acquisition (for example, LabView, Coach, LoggerPro, DataMate)
Suggestions for use:
The analysis of heating and cooling in different environment conditions is performed by using a ceramic heating element (see figure 4_5a)).

Figure 4_5a)
As a first step, the resistance is secured to a rigid support set on a teflon base, where all the electrical connections (rigid wires) that are necessary both to the heating of the resistance and the reading of the temperature values are also anchored (see Fig. 4_5b) and Fig. 4_5c)).

As a second step, the edge of a vacuum bell is greased by using vacuum grease, so that it can adhere well to the base (generally a glass disk), eliminating all the inevitable roughness that would not allow a good seal of the vacuum.

Figure 4_5d)The resistance is then enclosed into the bell which is resting on a base which in turn is resting on a metal platform in order to provide high rigidity.

Figure 4

Then, we are ready to turn on the pump starting the vacuum procedure.

Figure 5

This phase needs some time (usually 30/40 min) in order to obtain a vacuum of about 0.1 mbar. The bell is, then, isolated from the pump with a special valve and the pump can be turned off.

Figure 6

The heating of the resistance is obtained by simply turning on the current (Joule effect) and the values of temperature are directly obtained from the output signal of the thermocouple placed in contact with the resistance. Such a signal is suitably amplified and calibrated by the Fluke module which provides values of voltages in mV directly interpreted as Celsius degrees (1mV/1°C).

Figure 7

The measures to be carried out for the three different conditions (radiation, free convection and forced convection) are performed on the base of the following protocol. For each condition, the resistance is heated by using the same electrical power (for example 8 W), so we are able to compare the maximum temperatures reached (the plateaux of curves in Figure 10). Then, the current is turned off and the cooling process is analysed.

Figure 8

The same procedure can be performed allowing air to enter up to atmospheric pressure and then doing the measurement in free convection and in forced convection without the bell and with a fan placed in the direction of the resistor just during the cooling phase.

Figure 9
The resultsThe following graph shows the results at constant power in the three experimental situations:

Figure 10

It can be observed that the equilibrium temperatures of equilibrium are very close for heating in a vacuum and in open air and differ significantly with respect to the forced convection.The following figure represents only the cooling processes and allows us to easily compare the different cooling rate in the three different situations.

Figure 11
Figure 11 shows that the cooling rate for the natural convention and irradiation are almost the same, whereas the cooling rate in the case of the forced convection is much greater.

Possible questions: