AECL, Whiteshell Laboratories
Pinawa, Manitoba, Canada
Experimental and numerical studies have been performed to determine the heat transfer coefficients from a finned cylindrical surfaces to subcooled boiling water. Tests were conducted in an annular test section consisting of a heater with either six, eight or ten longitudinal, rectangular fins. The heater was made from a stainless steel tube coated with aluminum oxide and clad with an outer aluminum sheath. The axial heat flux is uniformly distributed along the heater. Tests were carried out at pressures between 110 and 350 kPa, velocities between the 1 and 10 m/s, and inlet water temperatures 26 to 90$\deg$ C.
A two-dimensional finite-element heat transfer model using the Galerkin method was used to analyze the experimental data to obtain heat transfer coefficients along the finned surface. The power temperature, pressure, velocity and fluid temperature measurements were supplied to the model for each experimental data point to be analyzed. Two different procedures were used to determine heat transfer coefficients around the finned periphery.
The first method was applied for the partial subcooled boiling where single phase- and boiling regions were present along the finned periphery. The experimental data points having the fin-tip temperature less than the ONB (onset of nucleate boiling) temperature and a fin root temperature greater than the ONB temperature were used. The single-phase heat transfer coefficient was applied along the finned surface from the ONB point to the fin tip. The heat transfer coefficient was altered along the boiling region until the predicted temperature was matched by its measured value. The second method was applied for tests having the measured fin-tip and fin-root temperatures above the ONB temperature (fully developed subcooled boiling). The solution procedure was to alter the heat transfer coefficient along the finned surface until the predicted surface temperature matched the measured fin-root and fin-tip temperatures.
A new correlation was developed based on assumption that the total surface heat flux around the periphery is made up of a nucleate boiling contribution and single-phase forced convective contribution. Multiple regression analysis was used to give the values of the constant and exponents in the proposed correlation. Good agreement has been found between heat transfer coefficients obtained from the numerical analysis and those predicted by the correlation.