Salem A. Al-Dini

Stability of electrohydrodynamic induction pumping of liquid film in vertical annular configuration

Instability of electrohydrodynamic (EHD) induction pumps can manifest itself in a sudden drop/jump in pump output. The instability can also result in alternating/bidirectional flow. To understand and avoid this erratic behavior of the pump operation, a nondimensional stability analysis of EHD induction pumping of liquid film in a vertical annular configuration in the presence of an external load (i.e., pressure gradient and gravitational force) for repulsion mode is carried out. A general nondimensional stability criterion is presented, indicating that the stability of the pump depends on the nondimensional geometric parameters of the pump as well as the nondimensional electric properties of the liquid film. A stability map based on dimensionless electric conductivity and liquid-film thickness is presented. The effect of the dimensionless angular velocity on the nondimensional interfacial velocity under the influence of a pressure gradient and gravitational force is investigated. It is also shown that the erratic behavior of the unstable pump can be eliminated by a proper selection of geometric and liquid-film parameters, as well as the traveling electric-wave frequency.

Effectiveness-NTU Relations for Parallel Flow Heat Exchangers Subjected to Heat Leak from Outside

The effectiveness-NTU relations are a valuable piece of information for designing and rating of heat exchangers when heat leaks are significant. In this paper, the closed-form relations for parallel flow heat exchangers when the heat leak is either on the cold or hot side of the heat exchanger are presented. The results are also presented in a graphical form to illustrate dependence of the fluid capacity ratio as well as the dimensionless heat leak parameter. It is demonstrated that when the dimensionless heat leak term approaches zero, the solution reduces to the classical effectiveness-NTU relations for parallel-flow configurations. In addition, when the ratio approaches zero, the analytical solutions reduce to the cases of condenser and evaporator applications.

Investigation into thermal performance of nanosized phase change material (PCM) in microchannel flow

Purpose – Heat capacity enhancement is important for variety of applications, including microchannel cooling and solar thermal energy conversion. A promising method to enhance heat capacity of a fluid is by introducing phase change particles in a flow system. The purpose of this paper is to investigate heat capacity enhancement in a microchannel flow with the presence of phase change material (PCM) particles.Design/methodology/approach – Discrete phase model (DPM) and homogeneous model have been compared in this study. Water is used as the carrier fluid and lauric acid as the PCM particles with different volume concentrations, ranging from 0 to 10%. Both the models neglect the particle‐particle interaction effects of PCM particles.Findings – The DPM indicates that presence of 10% volume concentration of PCM particles does not cause an increase in the pressure drop along the channel length. However, prediction from the homogeneous model shows an increase in the pressure drop due to the addition of nanopart...

Heat Transfer Enhancement in Microchannel Flow: Presence of Microparticles in a Fluid

In the present study, a numerical model was developed for laminar flow in a microchannel with a suspension of microsized phase change material (PCM) particles. In the model, the carrier fluid and the particles are simultaneously present, and the mass, momentum, and energy equations are solved for both the fluid and particles. The particles are injected into the fluid at the inlet at a temperature equal to the temperature of the carrier fluid. A constant heat flux is applied at the bottom wall. The temperature distribution and pressure drop in the microchannel flow were predicted for lauric acid microparticles in water with volume fractions ranging from 0 to 8%. The particles show heat transfer enhancements by decreasing the temperature distribution in the working fluid by 39% in a 1 mm long channel. Meanwhile, particle blockage in the flow passage was found to have a negligible effect on pressure drop in the range of volume fractions studied. This work is a first step towards providing insight into increasing heat transfer rates with phase change-based microparticles for applications in microchannel cooling and solar thermal systems.© 2010 ASME

Effectiveness–NTU Relations for Counterflow Heat Exchangers: The Effect of Kinetic Energy Variation and Heat Leak From Outside

The effectiveness–number of transfer units (NTU) relations are useful data for designing and performance evaluation of heat exchangers with fluids having considerable variation in velocities in the presence of heat leak. In this article, the closed-form (benchmark) solutions for counterflow heat exchangers, when the heat leak is either on the hot or cold side of the heat exchanger in the presence of kinetic energy variation, are presented. It was found that the effectiveness depends on NTU and fluid capacity ratio along with six other dimensionless variables that reflect the magnitude and axial distribution of the kinetic energy and heat leak on the hot and cold sides of the heat exchanger. The results are also presented in a graphical form exhibiting the variation of effectiveness of the heat exchanger with the already-mentioned parameters. It was demonstrated that when the dimensionless heat leak and kinetic energy terms approach zero, the solution reduces to the classical effectiveness–NTU relations for counterflow heat exchangers.

The effect of thermal radiation on the heat transfer characteristics of lid-driven cavity with a moving surface

Purpose – There are industrial applications for varying speed lid-driven flow and heat transfer such as the float glass process where the glass film stretches or thickens depending on the desired thickness. Hence the tin cavity underneath or the nitrogen cavity above is being driven by a variable speed. The purpose of this paper is to simulate such behavior. Design/methodology/approach – Numerical solution of variable speed lid-driven cavity is carried out with thermal radiation being considered using control volume approach and staggered grid and applying the SIMPLE algorithm. Transient simulation is used for 2D model in the present study. Second order upwind schemes were used for discretization of momentum, energy equations and time. Findings – Under laminar conditions, thermal radiation plays a significant role in the heat transfer characteristics of the lid-driven cavity. This effect is more significant for blackbody radiation and decreases as the surface emissivity decreases. Nusselt number (Nu) beha...

Design of volumetric solar flow receivers

The development of efficient solar thermal receivers has received significant interest for thermal energy to electrical power conversion and heating applications. Volumetric receivers, where the incoming solar radiation is absorbed in a fluid volume, have advantages over state-of-the-art surface absorbers owing to the reduced heat losses at the surface. To efficiently distribute and store the thermal energy in the volume, nanoparticles can be suspended in the liquid medium to scatter and absorb the incoming radiation. In such systems, however, compact models are needed to design and optimize the performance. In this paper, we present an analytical model that can be used to perform parametric studies to investigate the effect of heat loss, particle distribution, and flow rate on receiver efficiency. The analytical model was formulated by modeling the suspended nanoparticles as embedded heat sources. The heat equation was solved with the surface heat losses modeled using convective losses based on Newton’s law of cooling. The analytical solution provides a convenient tool to predict two-dimensional temperature profiles for a variety of heat loss and inlet fluid temperature conditions. The efficiency of the receiver is defined as the ratio of the amount of thermal energy transported by the fluid to the total incident solar energy. For very large lengths the thermal energy carried by the fluid reaches a maximum steady value as the amount of heat loss equals the incident solar energy. The model can be used to estimate the approximate receiver lengths required to achieve near peak bulk fluid temperature. The results from this study will help guide experimental design, as well as practical flow receivers for solar thermal systems. Predictions made on a channel of 1mm depth with a solar concentration of 1 show that there exists a maximum system efficiency of 0.3373 for a dimensionless receiver length of 1.66.Copyright