Fluid Flow and Heat Transfer in Cavities with Inlet and Outlet Ports: Effect of Flow Oscillation and Application to Design of Microvalves

Abstract

A Computational Fluid Dynamics (CFD) code is developed and tested to solve the two-dimensional form of the Navier-Stokes and energy equations. The flow field and heat transfer in a square cavity with inlet and outlet ports is investigated thoroughly. The effect of introducing a constant and oscillating inlet velocity on the forced convection in a square cavity is studied. As a practical application of the generic problems studied, a three-dimensional model of a piezoelectrically-actuated microvalve is presented. The effects of the Reynolds number, the ports width, and position of the outlet port on the steady-state fluid flow and heat transfer in a sealed cavity have been studied in great detail. The trends of the pressure drop, local and mean Nusselt numbers are elucidated for a wide range of operating conditions. It is concluded that placing the outlet port in parallel to the inlet port reduces the pressure drop significantly, whereas placing the outlet port at the corners of the cavity increases the heat transfer rate in the cavity. The transient evolution and periodic characteristic of the flow in a cavity with an oscillating inlet velocity have been studied. To maximize heat transfer and minimize pressure drop, the location of the inlet and outlet ports are chosen based on the result of the previous study. A detailed discussion of the effects of the Reynolds number and the frequency of the oscillating velocity is presented. The results indicate that both the flow and thermal fields reach their periodic states after a certain period of time. The results show that heat transfer rate greatly increases when the period of the oscillating inlet velocity is close to convective time scale of the cavity (St ˜ 1). Computational modeling of liquid flow in a NASA JPL (Jet Propulsion Library) piezoelectrically-actuated microvalve is discussed. The three-dimensional velocity and pressure fields are obtained for different deflections. By changing the mass flow rate at the inlet, the pressure drop between the inlet and outlet ports is found and a loss coefficient is determined for every deflection. The predicted pressure drop values are compared to the experimental data for water flow within the microvalve. A correlation is presented for the mass flow rate versus the pressure drop coefficient. The results show that the pressure drop increases exponentially by decrease of the deflection.