The experimental research on heat transfer coefficients of low temperature and low Reynolds number nitrogen gas

Abstract

The aim of this experimental research is to study the performance of heat transfer and pressure drop in the internally finned heat exchanger with low-velocity nitrogen gas flow at low temperature, which is often used in low temperature liquid vessel to reduce cool loss. In this experiment, a special structure had been designed to get the fitting test data. A 200 mL liquid nitrogen tank was installed at the bottom of the system, which provided low velocity and low temperature nitrogen gas. The heat exchanger was installed in the outlet pipe of nitrogen gas. There were heating film installed at the bottom of liquid nitrogen tank and heating wire installed on the surface of the heat exchanger to get the different inlet nitrogen gas velocity, temperature and different heat exchanger temperature. Both the liquid nitrogen tank and heat exchanger were placed in a vacuum vessel. In this way, the heat exchanger cannot perform convection with the air but can only convective heat with the nitrogen gas. The nitrogen gas temperature was measured by diode thermal sensor with using special package and installment in order to get the accurate temperature. A flow meter was installed at the end of the nitrogen gas outlet pipe to measure the gas flow rate in the heat exchanger. With the temperature and the flow rate been measured, the heat transfer coefficients can be obtained as h = C p × Q m × T out − T in A × ( T s − T fluid ) , where h is heat transfer coefficients, A is heat exchanger area, T s is heat exchanger temperature, T fluid is average fluid temperature in the heat exchanger, C p is fluid isobaric specificheat, Q m is fluid mass flow rate, T in is inlet fluid temperature and T out is outlet fluid temperature. Also, the pressure drop can be obtained as h f = δ × u 2 2 g , h m = λ × l d × u 2 2 g , where l is pipe length, d is pipe diameter, u is average velocity, g is gravitational acceleration, λ is on-way resistance coefficient and δ is local resistance coefficient. In this paper, the heat exchanger was studied with nitrogen gas temperature in the range of 90 to 200 K, velocity 0.03 to 0.60 m/s. The Reynolds number is small, ranging from 0 to 100. The pressure drop was calculated about several hundred Pascal, which is very small compared with the nitrogen outlet pressure (101325 Pa). So the analysis focuses most on the property of heat transfer coefficients. From the test data, the relationship between heat transfer coefficient value and gas velocity shows linearly direct ratio when inlet nitrogen gas temperature was 105 K and heat exchanger temperature was between 150 to 180 K; The relationship between heat transfer coefficient value and heat exchanger temperature also shows linearly direct ratio when inlet nitrogen gas temperature was 115 K and velocity was 0.09 m/s. The Nusselt numbers, Reynolds numbers and Prandtl number were calculated and a dimensionless correlation can be obtained as N u = 0.0060 × R e 1.0239 × P r − 0.3367 , R e ∈ ( 0 , 100 ) . The experimental error had been taken into account. Using the error transfer formula, the relative experimental error of Nusselt numbers and Reynolds numbers number were ±4.86% and ±2.86%. This experiment studies the performance of heat transfer and pressure drop in the internally finned heat exchanger with nitrogen gas temperature 90 to 200 K and velocity 0.03 to 0.60 m/s. The heat transfer coefficients of different nitrogen gas temperature and different flow rate had been obtained to form a dimensionless correlation. And this dimensionless correlation can be used for designing heat exchanger especially when nitrogen gas temperature between 90 and 200 K with low Reynolds number.