Convective Transport in Channels due to the Combined Effect of Shear

and Imposed Pressure

Yogesh Jaluria

Board of Governors Professor & Distinguished Professor

Department of Mechanical and Aerospace Engineering,

Rutgers University, Piscataway, NJ 098854, USA

Email: jaluria@soe.rutgers.edu

ABSTRACT

The flow and convective heat transfer in channels are generally due to an imposed pressure difference. Flows in mini- and microchannels for thermal management of electronic systems and in heat exchangers are examples of such flows. However, in many important cases, the shear due to a moving surface may also act in conjunction with the pressure, resulting in both aiding and opposing circumstances. Examples of such flows are seen in lubrication and in manufacturing processes like extrusion, coating, and wire drawing. In optical fiber drawing and coating processes, for instance, the moving fiber imparts shear along with the imposed pressure. The transport in the channels strongly influences the thermal processing of the material and the final product. Similarly, cooling of optical fibers after the furnace drawing process is another important step in the overall fiber fabrication process. The shear is imparted by the moving fiber and inert gases like Helium and Nitrogen are driven by pressure into the cooling channel. In extrusion processes as well, shear and pressure driven flows arise and affect the transport mechanisms that influence the thermal processing of the extruded material. This paper is focused on such processes, where the flow and the convective heat transfer in channels are induced by both shear and pressure. Of particular interest are mini- and microchannels, though larger channels are also considered. The transport processes at the inlet and outlet regions of the channels are of special intertest and are discussed in detail. Experimental and numerical results are presented to describe the flow in the channel and the resulting convective heat transfer. The increase in pressure in channels with reducing diameter or width is determined. This is of interest in dies and extrusion processes. It is seen that, in several practical circumstances, high velocities and fluid viscosity result in greater shear-induced pressures than the imposed pressure. The flow is then dominated by the shear effects due to the moving surface. The flow in narrow channels often develops very rapidly, resulting in largely developed flow regions. Thus, the transport rates are relatively small over much of the flow region. Methods to enhance the heat transfer under these circumstances by disturbing the flow are outlined. Comparisons between experimental and numerical results show good agreement. Therefore, the validity of the numerical models for these processes is established. The results obtained can also be used for the design of the thermal systems, particularly in lubrication, materials processing, and manufacturing.