Unsteady-State Heat Transfer to and from Gases in Laminar Flow

In this investigation of the problem of unsteady-state heat transfer to and from gas, experiments have been carried out with the flow of air through a passage of circular cross-section. Heat transfer takes place within a single vertical tube, around which is an annular guard space. Because of the magnitude of heat losses at lower values, the investigation is confined to values of Reynolds number between 1,000 and 2,500. Within this range, at moderate temperature differences, the processes of heating and cooling of gases reveal similar heat-transfer properties. When the tube wall-temperature is either constant or has a uniform temperature gradient, it is shown that the Nusselt number is in agreement with that predicted for steady-state heat transfer for the same wall-temperature distribution.

The investigation deals with single-cycle operation referred to as ‘single blow’, in which the supply is either hot or cold, and with cyclic operation as a parallel flow regenerator. Heat-transfer coefficients obtained from the cyclic operation tests are of the same order as those from the single-blow tests.

An analytical expression for the prediction of outlet temperatures, assuming a varying supply inlet-temperature and non-uniform initial wall-temperature, is presented and this is used in the calculation of the single-blow results.