treated as the measured exit fluid bulk temperature (Tb). Local Tb along the two-pass channel in streamwise direction is calculated using the enthalpy balance method. The Tb increase between two successive axial spots where Tw are measured is estimated as (qfdA)/(m_Cp) where qf, dA and Cp respectively stand for the local convective heat flux, segmental heating area and the constant pressure specific heat of the test coolant. Such calculation procedure starts from the measured fluid entry temperature with the calculated and the measured exit fluid Tb compared. Raw data are collected when the differences between the calculated and measured exit fluid Tb are less than 10%. With the confirmation by the measured exit fluid Tb, the calculated local Tb is selected as the reference temperature to determine local Nu and all the temperature dependent fluid properties required for data processing. Nevertheless, with present Tb range of 296.32–358.24 K, the maximum variation in Prandtl number (Pr) is about 1.7% so that the Pr effect is exclusive from present investigation.
The present IR thermo-graphic system detects the full-field Tw distributions at the scan rate of 60 Hz with the maximum percentage of uncertainties within +2%. Each Tw distribution over the ribbed heating foil (1) is scanned by the infrared radiometer which takes 60 frames of 239 255 matrix in one second. To minimize the background reflection during each Tw scan and the external heat loss from the heat transfer test module, a shield envelops the infrared camera and the heat transfer test module which is warped by thermal insulation fiber. With present optical IR setup, the infrared emission from the heating foil is normal to the plane of the focal lens. Prior to heat transfer measurements, the emissivity over the temperature range of 290–400 K is determined as 0.88 via the calibrating tests for present test configurations to ensure the maximum discrepancies between the calibrated thermocouple and the IR measurements are less than +0.3 K. The com- mercialized on-line monitoring program, which enables the control for scanning, collecting and transmitting IR signals for on-line monitoring or post data processing, is installed in the PC at the control console.