Numerical investigation of the effect of microstructures on heat transfer and fluid mixing in microchannels

Master's Thesis in Mechanical Engineering (Energy Conversion)

Abstract

Numerical investigation of the effect of microstructures on heat transfer and fluid mixing in microchannels

The need to transfer high heat from electronic packages and optical systems etc. has created a challenge in the field of thermal management. In this field, microchannel heat wells play a great role as a key factor in increasing the thermal dispersion power of systems in small dimensions. In this study, three-dimensional smooth flow of cooling water with forced convection heat transfer inside microchannels with microstructures has been investigated. These microstructures have a circular cross-section and their dimensions are fixed, and they are embedded in the bottom of the channel in the form of arrays one in the middle or with a diagonal pattern. The governing equations include the equations of conservation of mass, momentum and energy. In this research, Fluent software was used to study the microchannel heat well. After the accuracy of the obtained results compared to the laboratory data was ensured, the effect of microstructures on the heat transfer performance of the thermal well was studied. The results show that at pumping powers greater than 0.5 W, the heat transfer rate of finned microchannel heat well is lower than a simple but optimal microchannel heat well. But in small pumping power such as 0.05 W, the heat transferred from the optimal simple microchannels is lower than its value for the finned heat well. Also, the rate of entropy generation in finned microchannels was investigated and the dimensionless entropy generation rate was determined as a suitable factor for optimizing finned microchannel heat wells. Among other studies carried out in this research is the investigation of the effect of different arrangements of cylindrical vanes in microchannels on the mixing phenomenon in small dimensions. The results show high mixing efficiency in finned microchannels with one-in-one arrangement for different Reynolds numbers. At the end, the temporal flow around the vanes inside the channel was studied. style="direction: rtl;">One ??of the important issues in mechanical engineering is heat transfer in heat exchangers and microchannels. Today, with the development of technology, the need to design efficient converters is considered a necessity. This is while rapid progress has been made during the last decade in the field of production and use of high-power micro-devices, which shows the need for a comprehensive and detailed investigation of the basic aspects of fluid flow and heat transfer at the micro scale, and has drawn much attention to the problems of fluid mechanics at the micron scale. All the efforts of active designers and researchers in this field have been to increase the heat exchange and finally improve the efficiency of the whole system. Among the measures that can be taken in this field is the use of internal surfaces or blades. Fins increase the surface of heat transfer and finally, if they are well designed, they can significantly increase the efficiency of microchannels. Fins are widely used in the industry, including fins for cooling computer processors and electronic components. Today, with the increasing progress of computers and the arrival of powerful processors and supercomputers, a large amount of information is processed in a very short time. Fast processing causes heat to be generated in the processor and if this heat is not removed, thermal stress will cause the processor and ultimately the entire system to be destroyed. In today's world and with the construction of supercomputers, the heat removal technique must be more efficient and effective than in the past. Therefore, the need to design thermal wells with higher efficiency is clearly felt.   

The process of heat transfer caused by the fluid flow in the channels plays a major role in the operation of many natural systems and man-made systems. Channels with a diameter between 3 mm and 200 micrometers are called microchannels.As we know, the rate of the heat and mass transfer process depends on the side surface of the channel (it has a direct relationship with D) and the flow rate depends on the cross-sectional area of ??the channel (it has a direct relationship with ²D), so the smaller the diameter of the channel (D), the higher the ratio of the side surface to the volumetric flow rate. This property has also been used in the human body. In the lungs and kidneys, there are channels that shrink in diameter and reach about 4 micrometers, and we have the most efficiency in the heat and mass transfer process in these two organs of the body. The presence of micro channels in nature should be seen in kidneys, lungs, brain, intestines and veins. searched Meanwhile, there are various microchannels in man-made systems such as some heat exchangers, nuclear reactors, air separation units and blood and DNA analyzers [1].

Using thermal wells with microchannel channels[1] has led to many improvements in low thermal resistance, compact structure, low cooling fluid rate, uniform temperature distribution in the flow direction, etc. In this research, due to the importance of microchannels in cooling systems, the increase of heat transfer in microchannels will be investigated by considering microstructures or grooves in the inner body of microchannels. For this purpose, in the current research, a three-dimensional mathematical model will be introduced for solid and liquid combined heat transfer [2]. The Navier-Stokes and energy equations for the liquid region and the energy equation for the solid part are solved at the same time, and the parameters of pressure drop along with heat transfer in a thermal well[3] including single-phase microchannels are investigated.

In fluid mechanics and heat transfer, the issue of reverse flows and flow separation in different geometries is of great importance because the existence of these areas has a great effect on the force from the fluid to the surface and the pressure drop. Also, heat transfer in these areas is very important because the presence of return areas changes the value of the heat transfer coefficient and causes it to be maximized in the place where the separated fluid sticks to the surface again.

The type of incompressible fluid flow regime is a function of the Reynolds number. The Reynolds number represents the ratio of the inertial force to the viscous friction force. (1-1) This number is usually expressed in terms of suitable parameters of the flow and its geometry. For microchannels with cylindrical fins, the Reynolds number is defined based on the diameter of the circle, and for the rest of the microchannels, it is defined according to the hydraulic diameter of the channel and the average flow velocity in the microchannel. The flow inside the microchannels is generally in the range of low Reynolds numbers due to the small hydraulic diameter of the channels and also the low fluid velocity. This is while in the microchannels with microstructures, when the fluid hits the vanes, a return flow is formed behind them, and there is also the possibility of flow separation in these channels. It cannot be said for sure at what Reynolds number the flow becomes smooth and turbulent in microchannels with microstructures. As the Reynolds number increases, vortices begin to oscillate around a stable point, such as backflow behind a cylinder or vanes in microchannels. In this case, the flow is still calm and random disturbances and disorder are not observed. The frequency of such oscillations is defined by the Strouhal number, which is dimensionless, as follows: (2-1) The Strouhal number is the ratio of the characteristic time of the current ( ) to the period of the oscillations ( ), where ƒ is the frequency of the current oscillations. In most situations of the flow, such as the backflow behind the cylinder, vane or sphere, the Strouhal number is in the range of 0.2. As the current increases, the alternating oscillations become unstable and irregular. More and smaller vortices are formed, which can only be described by statistical methods.