Heat transfer and pressure drop in forced flow helium II at high Reynolds numbers

摘要

An experiment has been built to investigate heat transfer in He II forced flow of velocities up to 22 m/s, which is an order of magnitude larger than in previous experiments. Pressure drop, steady-state and transient heat transfer have been studied in three 10 mm ID strait smooth test sections of length around 1 m. The linear pressure drop in the test sections increases with the square of the flow velocity. These pressure drops, which therefore become significant at high velocities, represent an isenthalpic expansion resulting in temperature increases along the flow path (Joule-Thomson effect). These initial temperature gradients due to the flow alone are the basis on which heat transfer from external sources can be added.;Steady-state density powers up to 16 W/cm2 of channel cross section were applied near the middle of one of the test sections while the temperature measured at several locations upstream and downstream. These measurements have been compared with a steady-state numerical model which doesn't include pressure effects and is commonly used for the modelisation of counterflow heat transfer in forced flow. The agreement is good between the experimental and numerical results for the lowest velocities confirming the appropriateness of this model in that case. The comparison for higher flow velocities (larger than around 3 m/s) shows the need to include pressure effects in the heat transfer for forced flow at intermediate and high velocities.;Transient heat pulses of duration between 1 and 20 ms and of power density between 9 and 40 W/cm2 were also applied in one of the test sections. The temperature was measured at several locations downstream as the pulses were carried by the forced flow and their shape transformed by counterflow heat transfer. A transient numerical model including pressure effects was developed and compared with the experimental results. The agreement is very good for the lowest and highest velocities (lower than around 4 m/s and higher than around 12 m/s depending on the heat flux). The agreement is not good for intermediate velocities. Observations of the areas of disagreement suggest the need to modify the heat flux expression used when the heat flux is near zero.