Chen-Ru Zhao

Influence of Flow Acceleration on Convection Heat Transfer of CO2 at Supercritical Pressures and Air in a Vertical Micro Tube

The convection heat transfer of CO2 at supercritical pressures in a 0.0992 mm diameter vertical tube at relatively high Reynolds numbers (Rein = 6500), various heat fluxes and flow directions are investigated experimentally and numerically. The effects of buoyancy and flow acceleration resulting from the dramatic property variations are studied. The Results show that the local wall temperature varied non-linearly for both upward and downward flow when the heat flux was high. The difference in the local wall temperature between upward and downward flow is very small when the other test conditions are held the same, which indicates that for supercritical CO2 flowing in a micro tube as employed in this study, the buoyancy effect on the convection heat transfer is insignificant and the flow acceleration induced by the axial density variation with temperature is the main factor leading to the abnormal local wall temperature distribution at high heat fluxes. The predicted temperatures using the LB low Reynolds number turbulence model correspond well with the measured data. To further study the influence of flow acceleration on the convection heat transfer, air is also used as the working fluid to numerically investigate the fluid flow and heat transfer in the vertical micro tube. The results show that the effect of compressibility on the fluid flow and heat transfer of air in the vertical micro tube is significant but that the influence of thermal flow acceleration on convection heat transfer of air in a vertical micro tube is insignificant.Copyright

Convection Heat Transfer of CO2 at Supercritical Pressures in a Vertical Mini Tube

This paper presents the experimental and numerical investigation results of the convection heat transfer of CO2 at supercritical pressures in a 0.0992 mm diameter vertical tube at various inlet Reynolds numbers, heat fluxes and flow directions. The effects of buoyancy and flow acceleration resulted from the dramatic properties variation were investigated. Results showed that the local wall temperature varied non-linearly for both upward and downward flow when the heat flux was high. The difference of the local wall temperature between upward flow and downward flow was very small when other test conditions were held the same, which indicates that for supercritical CO2 flowing in a mini tube as employed in this study, the buoyancy effect on the convection heat transfer was quite insignificant, and the flow acceleration induced by the axial density variation with temperature was the main factor that lead to the abnormal local wall temperature distribution at high heat fluxes. The predicted values using the LB low Reynolds number turbulence model correspond well with the measured data. Velocity profiles and turbulence kinetic energy near the wall varying along the tube generated by the numerical simulations were presented to develop a better understanding.© 2009 ASME

Predictions of In-Tube Cooling Heat Transfer Coefficients and Pressure Drops of CO2 and Lubricating Oil Mixture at Supercritical Pressures

ABSTRACT The in-tube cooling heat transfer and flow characteristics of supercritical pressure CO2 mixed with small amounts of lubricating oil differ from those for pure CO2 due to the entrainment of the lubricating oil as well as the sharp property variations of the supercritical CO2 working fluid. In-tube gas cooling flow and heat transfer models were developed in this study for CO2 with entrained polyol ester type lubricating oil in a CO2 gas cooler at supercritical pressures. A “thermodynamic approach,” which treats the CO2–oil mixture as a homogenous mixture was used with the heat transfer coefficients and frictional pressure drops evaluated based on the thermophysical properties of the CO2–oil mixture. Thermophysical property variation correction terms as a function of the wall temperature and the oil concentration were included in the models. The frictional pressure drop correlation predicts more than 90% of the experimentally measured data within ±10%, while the heat transfer coefficient correlation predicts more than 90% of the experimentally measured data within ±20%.

Study of the Numerical Solving Methods of the Motion Model of Polydispersed Droplets

Moisture separation is one of crucial devices in PWR power plant for it plays an irreplaceable role in eliminating droplets from steam and supplying dry-saturated for turbines. It would be helpful to design and optimize the structure of moisture separator through analyzing the behaviors of droplets. This paper studies the numerical solving methods of the motion model of polydispersed droplets. The equations of model belong to the field of nonlinear stiff ordinary differential equations, thus backward differentiation formula, a kind of multi-steps methods are used which advance in solving stiff different equations. The coefficients of equations involve the velocity and rotation of flow field in separator, and these parameters are given by the output of Fluent. After solving the differential equations we can get the velocity and angle velocity of droplets in different locations and the movement track of droplets in separator, as well as the separation efficiency of moisture separator for polydispeased droplets with special distribution. Finally through the numerical solving of the motion model of polydispersed droplets in chevron-type separator, we find that multi-steps method improves the numerical stability and reduces the steps of iteration under the same calculation precision compared with classical methods such as fourth-order Runge-Kutta method. So it lays the foundation for development of moisture separator program suit for engineering computing.Copyright

Predictions of Heat Transfer Coefficients and Pressure Drops to Supercritical Pressure HCFC22 Flowing in a Small Tube

In this paper, experimental flow and heat transfer data of supercritical pressure HCFC22 flowing in a uniformly heated smooth tube with inner diameter of 1.004 mm at p/pc=1.1 obtained by the authors are analyzed accounting for the influence of the thermophysical properties variation, the buoyancy effect, as well as the flow acceleration effect due to thermal expansion. These analyses indicate that both of the sharp thermophysical properties variation in the fluid adjacent to the wall with low density, low specific heat and low thermal conductivity and the flow acceleration effect due to thermal expansion have significant negative effects on the heat transfer under the present study conditions for HCFC22, while for the friction factor, the thermophysical properties variation is the predominant factor. The buoyancy effect on the flow and heat transfer is negligible.A new semi-empirical local heat transfer correlation accounting for the thermophysical properties variation and the flow acceleration effect due to thermal expansion for supercritical pressure fluids flowing through a vertical small tube during heating is proposed. The predicted values agree with 95% of the measured data within ±25%. In addition, a flow correlation with thermophysical properties variation correction terms to predict the friction factors for supercritical pressure fluids is proposed which predicts the measured friction factors within ±25%.© 2014 ASME

Convection heat transfer of CO{sub 2} at supercritical pressures in a vertical mini tube at relatively low reynolds numbers

Convection heat transfer of CO{sub 2} at supercritical pressures in a 0.27 mm diameter vertical mini tube was investigated experimentally and numerically for upward and downward flows at relatively low inlet Reynolds numbers (2900 and 1900). The effects of inlet temperature, pressure, mass flow rate, heat flux, flow direction, buoyancy and flow acceleration on the convection heat transfer were investigated. For inlet Reynolds numbers less than 2.9 x 10{sup 3}, the local wall temperature varies non-linearly for both flow directions at high heat fluxes (113 kW/m{sup 2}). For the mini tube used in the current study, the buoyancy effect is normally low even when the heating is relatively strong, while the flow acceleration due to heating can strongly influence the turbulence and reduce the heat transfer for high heat fluxes. For relatively low Reynolds numbers (Re{sub in} {<=} 2.9 x 10{sup 3}) and the low heat flux (30.0 kW/m{sup 2}) the predicted values using the LB low Reynolds number correspond well with the measured data. However, for the high heat flux (113 kW/m{sup 2}), the predicted values do not correspond well with the measured data due to the influence of the flow acceleration on the turbulence. (author)