Experimental research of the effect of surface orientation on the subcooled flow nucleate boiling of water at low pressure

Abstract

The toughening emission standards and the costs saving requirements are pushing to the limits the design of compact heat exchangers in the automotive industry, meaning that today’s heat exchangers need to operate with a controlled level of boiling in their coolant side. Most of the experimental literature available tackle boiling in horizontal flat plates or vertical tubes, while the information regarding other orientations is much scarcer. However, in a compact heat exchanger all orientations are present therefore orientation parametrizations in boiling models are particularly important since upper-heating orientations have a strong influence on heat transfer mechanism and the critical heat flux due to the cancelation of the bubbles floatability forces that help their departure. Therefore the limiting heat flux in those parts is generally governed at boiling orientations not determined with precision. The experimental work presented in this paper analyses the dependence of the heat transfer coefficient with the inclination of the heated surface under subcooled boiling regime. Due to the heating method selected, the test part consists in an AISI 316 thin strip with a thickness of 0.5 mm brazed to a copper base, to ensure an industry-like heat exchange material as primary surface but avoiding unmanaged temperatures and heating powers. Experimental tests have been carried out on a single face heated rectangular channel under several operating conditions of bulk velocity, temperature, pressure and flow orientation: 0.1-0.9 (m/s) – 76.5-93.5 (°C) – 110-190 (kPa) – 0- 180 (°), to cover some of the most common conditions found in the automotive compact heat exchanger industry. The heat flux employed in each test ranged from 0.1 to more than 1 MW/m². After the data analysis some main dependences were identified and suggested that a global boiling model should include some parameters accounting for the relative orientation of the heated part and the coolant flow. This could be a valuable tool during the development of an automotive heat exchanger in which nucleate boiling is present.