Mingheng Shi

Three-Dimensional Numerical Simulation for Annular Condensation in Rectangular Microchannels

A three-dimensional model in rectangular microchannels with constant heat flux is developed to predict steady annular condensation. The condensate flow field on the side wall, which is dominated by surface tension, is divided into two regions: the thin–film region and the meniscus region. The momentum and mass equations, in both the vapor and meniscus regions, along with the film thickness equation in thin–film region are solved numerically. The distribution of the meniscus curvature radius, thickness of the condensate film, heat transfer coefficient, and wall temperature are all determined. The results indicate that with the development of condensation, the condensate in the thin–film assumes a convex profile shape at the side wall, with the crest located at the midpoint of the side wall. The film thickness in the thin-film region increases at upstream locations and decreases as the flow moves downstream. The average heat transfer coefficient in the thin-film region is much larger than that occurring in the meniscus region. And the highest local heat transfer coefficient occurs at the intersection of the thin-film region and the meniscus on a cross section where the maximum wall temperature exists. The circumferential average heat transfer coefficient decreases drastically upstream to a lower value. After that, it remains nearly constant until close to the end of the annular flow, where it again begins to decrease.

Optimal surface fractal dimension for heat and fluid flow in microchannels

The fractal Weierstrass–Mandelbrot function was introduced to characterize the multiscale self-affine rough surface of microchannels. Based on this fractal characterization, the role of the rough surface structure on the thermal and hydrodynamic properties in microchannels was evaluated using a computational fluid dynamic simulation. Once identified, these were used to determine the optimal surface dimension for heat and fluid flow. It was found that, no matter what the Reynolds number and roughness height are, the flow heat transfer performance is being optimized with increasing fractal dimension of the surface until to the dimension value of three (infinitely crumpled).

Slip boundary for fluid flow at rough solid surfaces

A molecular dynamics simulation of slip boundary for fluid flow past a solid surface incorporating roughness effect as characterized by fractal geometry has been conducted with a focus on the origin of slip, fluid structure, and slip boundary flow. The results indicate that interfacial slip develops provided that the wall is effectively uncorrugated. Compared with the atomically smooth surface, extra viscous dissipation is induced for shear flow past a rough surface and leading to a reduction in boundary slip. In particular, we find that a more irregular topography decreases the boundary slip even for the same statistical roughness height.

Thermal Characteristics of Heat Pipe with Axially Swallow-tailed Microgrooves

Abstract A thermal model for a heat pipe with axially swallow-tailed microgrooves is developed and analyzed numerically to predict the heat transfer capacity and total thermal resistance. The effect of heat load on the axial distribution of capillary radius, and the effect of working temperature and wick structure on the maximum heat transfer capability, as well as the effect of the heat load and working temperature on the total thermal resistance are all investigated and discussed. It is indicated that the meniscus radius increases non-linearly and slowly at the evaporator and adiabatic section along the axial direction, while increasing drastically at the beginning of the condenser section. The pressure difference in the vapor phase along the axial direction is much smaller than that in the liquid phase. In addition, the heat transfer capacity is deeply affected by the working temperature and the size of the wick. A groove wick structure with a wider groove base width and higher groove depth can enhance the heat transfer capability. The effect of the working temperature on the total thermal resistance is insignificant; however, the total thermal resistance shows dependence upon the heat load. In addition, the accuracy of the model is also verified by the experiment in this paper.

Study on boiling heat transfer in liquid saturated particle bed and fluidized bed

An investigation on the effects of solid particles on boiling heat transfer enhancement is performed. The range of particle diameter is from millimeter to nanometer. The experimental results show that boiling heat transfer can be enhanced greatly by adding the solid particle into the liquid whether in fixed particle bed or in fluidized particle bed. The boiling enhancement is closely related to the particle size, the initial bed depth and the heat flux applied. The experiments show that boiling characteristics are greatly changed when a particle layer is put on the heated surface. The major effects of fixed particle bed on nucleate pool boiling heat transfer are the nucleation, bubble moving and thermal conductivity effect. A boiling heat transfer correlation is obtained to predict the boiling heat transfer coefficients in a liquid saturated porous bed. A volumetric convection mechanism of boiling heat transfer enhancement by fluidized particles is proposed. The calculated results from the model suggested in this paper agree reasonably with the experimental values.

An in situ thermal response test for borehole heat exchangers of the ground-coupled heat pump system

Thermal response tests (TRTs) are crucial for the estimation of the ground thermal properties and thermal performance of the borehole heat exchanger (BHE) of the ground-coupled heat pump (GCHP) system. In this article, a TRT apparatus was designed and built to measure the temperature response of inlet and outlet sections of BHE in the test borehole, the apparatus can effectively operate under both constant heating flux modes and heat injection and extraction modes with a constant inlet temperature. A TRT for a project of GCHP located in the Jiangsu province of China was carried out by the experimental apparatus. Based on the experimental data, the heat transfer performances of BHE under heating and cooling modes were evaluated, and the ground thermal properties, which include the ground thermal conductivity, ground volumetric specific heat, borehole thermal resistance and effective soil thermal resistance, were determined by the line source model. The results indicate that the experimental device and analysis model proposed in this article can be effectively applied to estimate the ground thermal properties and thermal performance of BHE. During the process of thermal response of ground, the fluid temperatures vary acutely at the start-stage of 8 h, and then tend to be a steady state after 40 h. The test data during the start-stage should be discarded for improving the estimation accuracy of ground thermal properties. At the same time, the effective soil thermal resistance increases continuously with time and a steady-state value would be reached after the start-time, and this steady-state thermal resistance can be used to evaluate the required length of BHE. In addition, the heat transfer rate of the BHE under different operating conditions can be used for the further evaluation on long-term operation performance of GCHPs.

A Numerical Model for the Simulation of a Vertical U-Bend Ground Heat Exchanger Used in a Ground-Coupled Heat Pump

Heat transfer between the ground heat exchanger (GHE) and surrounding soil is a common problem for the optimum design and simulation of ground-coupled heat pump (GCHP). In this article, a quasi-three-dimensional numerical model of vertical U-bend GHE was proposed and developed by coupling the one-dimensional model of fluid in the depth direction with the two-dimensional heat transfer model of soil in the level direction but considering the soil temperature variation along depth. In the model, the soil in depth direction was divided into saturated and unsaturated regions by water table, and the influence of groundwater advection on the heat transfer performance of GHE was considered for the saturated soil region. Based on the model, the variations of heat rejection and fluid outlet temperature of GHE with time were discussed, and the influences of soil thermal properties and groundwater advection on the heat transfer performance of GHE were analyzed. The results show that the heat rejection in cooling mode will decrease with time, and an intermittent heat rejection mode can improve the heat transfer performance. Meanwhile, the increases of soil thermal conductivity and heat capacity can all enhance the heat transfer characteristics, and the exist of groundwater advection can improve the heat exchange between the GHE and surrounding soil. Furthermore, an experimental validation was performed in a solar-geothermal multifunctional heat pump experiment system. The results indicate the predicted values of heat rejection rate of GHE are agreed well with the corresponding measured data, and the guess average relative error for the daily heat rejection is about 5.5%. This means the numerical model proposed in this article is feasible and thus can be used to simulate the heat transfer of GHE.

Characterization of Surface Roughness Effects on Laminar Flow in Microchannels by Using Fractal Cantor Structures

The fractal characterization of the topography of rough surfaces by using Cantor set structures is introduced in this paper. Based on the fractal Cantor surface, a model of laminar flow in rough microchannels is developed and numerically analyzed to study the characterization of surface roughness effects on laminar flow. The effects of Reynolds number, relative roughness, and fractal dimension on laminar flow are all discussed. The results indicate that the presence of roughness leads to the form of the detachment, and eddy generation is observed at the shadow of the roughness elements. The pressure drop in the rough channel along the flow direction is no longer in a linear fashion and larger than that in the smooth channel. The fluctuation characteristic of pressure drop along the stream, which is due to the vortex formation at the wall, is found. Differing from the smooth channel, the Poiseuille number for laminar flow in rough microchannels is no longer only dependent on the cross-sectional shape of the channel, but also strongly influenced by the Reynolds number, relative roughness and fractal dimension of the surface.Copyright