Conference Year

2014

Keywords

pulsating flow, pulsation width modulate, evaporator control, heat transfer coefficient, two-phase flow regime.

Abstract

Pulsating flow can increase the heat transfer coefficient when a two-phase flow passes through a single horizontal tube through the following mechanisms: 1) When liquid-vapor two-phase flow and heat transfer occurs inside a tube, flow pulsations can change the flow pattern and increase liquid-wall contact area at a fixed void fraction, thus increasing the heat transfer coefficient; 2) Local pressure at some locations in the tube will drop more with pulsating flow, and the attendant reduction saturation temperature can increase local evaporation and heat transfer. However, the effects of pulsation are more complex for pulsations within a practical evaporator for which the use of a multi-tube construction and the presence of tube bends at the end of each tube certainly affect the resulting flow. In this case, it is anticipated that pulsation width modulation is very important for evaporator performance enhancement. Moreover, local pressure drop in the evaporator increases due to the pulsating flow, and this needs to be taken into account for practical evaporators when considering pulsation as a method of heat transfer performance enhancement. In this work, the effects of pulsation width on heat transfer coefficient and pressure drop in an air-to-refrigerant evaporator have been studied experimentally. An experimental system has been developed with two separate but identical evaporators located in separate but identical wind tunnels. The pulsation width can be controlled with a minimum period of 1s. Average heat transfer coefficient, local heat transfer coefficient and pressure drop have been measured and compared with and without pulsating flow. Two-phase flow regimes at different locations in the evaporator are also observed with pulsating flow, and a new flow regime characterization is provided. These observations are used to help explain the heat transfer enhancement in pulsating flow.

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