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.
Papers presented to the 12th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Costa de Sol, Spain on 11-13 July 2016.