Experimental Study of Flow Boiling Using R134a in Multi Microchannels

Abstract

This paper presents the results of an experimental investigation of flow boiling heat transfer in multi microchannel heat sink. The test section, 20 mm long and 15 mm wide, consisted of 26 rectangular microchannels (350 µm wide, 630 µm deep-i.e. Dh = 450 µm-and 200 µm wall thickness) produced by CNC machining from an oxygen free copper block. Using R134a as a test fluid, the experiments were conducted at a system pressure of 7 bar, heat flux range 7.6-625.8 kW/m 2 and mass flux range 50-300 kg/m 2 s. A high speed camera was used to capture the flow patterns. Four flow patterns were observed namely bubbly, slug flow/confined bubble, churn flow and annular flow as the heat flux gradually increased. Results show that the maximum heat transfer coefficient obtained is 35.52 kW/m 2 K.The results also show that the heat transfer coefficient increased with heat flux and there was no mass flux effect. Relevant literature correlations predicting the heat transfer coefficient were evaluated against the current data.

Experimental Investigation of the Heat Transfer and Pressure Drop inside Tubes and the Shell of a Minichannel Shell and Tube Type Heat Exchanger

“…Cold water was supplied directly from the water network in an open circuit (5). The flow of cold water was regulated using a valve ( 6) and its flow rate was measured by a rotameter (7).…”

“…In order to determine the value of the heat transfer coefficient it is necessary to know the temperature at the channel wall surface. At first sight, measuring the wall temperature inside a microchannel seems to be more problematic, but researchers have overcome this problem by creating microchannels at the end of a rectangular block made of a material characterised by a high thermal conductivity coefficient [5]. On the opposite side, the block was heated or cooled.…”

“…The heat exchange inside the investigated metal block was thus reduced to the case of one-dimensional heat conduction. By measuring the temperature gradient along the block, it was possible to estimate the microchannel wall temperature and the heat flow rate with high accuracy [5]. Applying a similar procedure is unfortunately impossible in the case of microtubes.…”