Experimental Research On Heat Transfer Characteristics Of Supercritical Pressure R134a In A Vertical Tube

In the organic trans-critical cycle?OTC?system,the organic fluid in the supercritical heater has no phase change during the heat-absorbing or heat-releasing process;therefore,the temperature variation curve of the fluid has a good thermal match with that of the finite heat source.For this reason,the OTC system has its superiority in the low-temperature waste heat recycling compared with traditional cycles.The heat transfer characteristics of organic fluids under supercritical pressures shows great difference with that of subcritical fluid and greatly influenced by the working conditions.On this account,it is necessary to carry out researches on the heat transfer characteristics of organic fluid under supercritical pressures under the working conditions of the supercritical heater in the OTC system.The experimental apparatus was designed and built,and the heat transfer characteristics of R134a under supercritical pressures was studied in the range of 4.3-4.8 MPa in pressure,500-1500kg?m^-2?s^-1 in mass flux,20-100 kW?m^-2 in heat flux,in both upward and downward flows.The result shows that:?1?within the experimental range,the heat transfer coefficient?HTC?increases and the difference between the HTCs of upward and downward flows decrease with the increase of mass flux.The peak value of HTC near the pseudo-critical temperature decreases with the increase of pressure when heat transfer deterioration does not happen,and may recover with the increase of pressure when heat transfer deterioration happens.The HTC decreases with the increase of heat flux in the low value range of heat flux,and recovers when the heat flux is high.?2?The dimensionless parameter Bu predicts the influences of buoyancy force on HTC fairly well.The influence of radical property variations plays a dominant role in the heat transfer process under the experimental conditions.?3?The new fitted heat transfer correlations have good prediction accuracies,and the mean deviation equals0.32%in upward flow and 0.77%in downward flow,the standard deviation equals9.6%in upward flow and 11%in downward flow.