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Title: Modelling, fabrication and testing of compound ribs for the enhancement of heat transfer in micro-heat exchangers
Authors: Wang, Haitao
Degree: Ph.D.
Issue Date: 2016
Abstract: Over the last few decades channels or ducts with artificial roughness in the form of ribs have been widely used in various scientific and industrial applications for controlling fluid flow in heat exchange device, such as heat exchangers, chemical reactors and ventilation, due to its higher effective heat transfer rate. Such ribs can generate turbulence and hence break up the viscous sub-layer near the heated surface, therefore reducing thermal resistance. The heat transfer and pressure drop are influenced by many factors, such as channel shape, hydraulic diameter and flow rate of fluid. In this thesis, both theoretically and experimentally studies were conducted to study the effect of geometrical configuration of two-dimensional transverse ribs with various compound micro-structures on the minimization of pressure drop in micro-heat exchangers. A novel compound ribbed surface was designed. The geometry of the compound rib was composed of a first-order transverse rib and a second-order micro-groove superimposed on the primary rib. The existence of second-order structures enhances heat transfer and increases flow friction as well. Therefore, it is highly desirable to optimize the trade-off between the performance of heat transfer enhancement and the effect of friction caused by the ribbed surface. A mathematical model was established to investigate the performance of heat transfer and the characteristics of flow dynamics in single-phase channels with various geometrical configurations of the two-dimensional (2D) ribs. To fully understand the effects of compound ribs on various flow rates, both laminar flow and turbulent flow were studied with a shear stress transport (SST) model. Numerical simulations were performed with Reynolds number (Re) ranging from 0 to 2,000 for laminar flow and from 20,000 to 60,000for turbulent flow. Based on the theoretical model and finite volume method, the 2D governing equations, such as the 2D continuity, incompressibility, momentum and energy equations were solved by computational fluid dynamic (CFD) software FLUENT. In contrast to the conventional ribs (i.e. symmetrical triangular rib, rectangular rib and semi-circular rib, etc.), the proposed compound rib can promote the reattachment of separated flow to the heat surface, thus shortens the distance from the attachment point to the rib and minimizes the reticulations zone. In the confined space, the application of compound ribs can minimize the pressure drop by reducing the pressure recirculation region at the leeward side of the ribs, as well as improve the performance of heat transfer. Since the configuration of the advanced compound ribs is difficult to machine by conventional methods, ultra-precision raster milling was used to generate these complex surface features. With the advantage of ultra-precision raster milling, a novel one-step machining process for the advanced compound rib is proposed and demonstrated. The present work investigated the principles of cutting strategy for the second-order structures and the cutting conditions that affect the basic features of the structures. Ultra-precision raster milling is usually time-consuming and costly. In order to improve the efficiency of the fabrication the complex compound rib without losing the quality and geometric accuracy of second-order structures, the effect of various machining factors on the quality of micro-groove was systematically studied, such as feed rate, depth of cut and spindle speed.
The cutting experiment found that the increase in feed rate greatly improves processing efficiency, but it also results in lower processing quality. The increase in feed rate leads to a rapid increase of cutting force, which aggravates the plastic deformation to the structure. Increased spindle speed can effectively enhance the cross-cutting effect of two adjacent cutting steps on removing material. However, higher spindle speed could cause the plastic deformation of the second-order micro-grooves for the small angle tool cutting. The cutting efficiency can be greatly improved by increasing the cutting depth, which also increases the cutting force on the second-order micro-grooves and results in their deformation. The experiment results suggested that the cutting depth is the most important factor affecting the quality of second-order micro-grooves. To improve the processing efficiency, it is significant to choose the right processing parameters. The effect of diamond cutting tool angle on the form accuracy of the compound rib is also reported. The compound ribs were successfully machined and installed on the cooling channels of micro-heat exchangers. In order to verify the predictions of numerical simulation, cooling experiments were designed and established for various configurations of the ribbed surface with two pitch lengths of the primary rib. The thermal performance was measured by the temperature decreasing rate and the average static temperature. Furthermore, the required pumping power was also measured. Good agreement was found between the simulation and experimental results. The maximum value of heat transfer rate for the optimized compound rib was found to be 2.8 times greater than that of a smooth surface. Compared to other optimized traditional surfaces which have 2D transverse ribs and better heat transfer performance as proved by previous researchers, the compound ribs improved the heat transfer rate by more than 6%. More importantly, by applying the compound ribs the pressure loss was reduced by 18%. In this thesis, the advanced functional compound ribs with micro-structures are successfully introduced. These ribs will not only enhance the heat transfer performance, but also minimize the pressure drop and further reduce the energy consumption of micro-heat exchangers. The work has provided essential basis for further optimization of the performance of micro-heat exchangers.
Subjects: Heat exchangers.
Heat -- Transmission.
Hong Kong Polytechnic University -- Dissertations
Pages: xxii, 218 pages : illustrations (some color)
Appears in Collections:Thesis

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