Ebrahim Al-Hajri

Experimental Characterization of a Nickel Alloy-Based Manifold-Microgroove Evaporator

Effective heat and mass exchangers are vital for further improvement of absorption cooling systems. In the current study, a novel manifold-microchannel evaporator was developed and tested. This paper reports heat transfer coefficients and pressure drop for a nickel alloy-based tubular microgrooved evaporator consisting of a novel manifold guided flow. The evaporator was designed for refrigerant-to-liquid heat exchange, and the heat transfer surface consisted of fine high-aspect-ratio microchannels having 100 μm channel width and 600 μm channel height. The refrigerant-side flow was guided through square manifold feeds with sides of 2 mm in length. A tube insert providing an annular gap of 2.5 mm was used on the water side. Experiments were conducted with R134a as the refrigerant for a flow rate range of 5–30 g/s and water-side flow rate range of 100–600 ml/s. An overall heat transfer coefficient of more than 10,000 W/m2-K was measured with a modest maximum pressure drop of 120 mbars and 100 mbars on the refrigerant and water sides, respectively.

Thermal Performance of Micro-Structured Evaporation Surfaces: Application to Cooling of High Flux Microelectronics

An experimental investigation on characterization of copper-finned micro-grooved surfaces for effective evaporation heat transfer with applications to cooling of high flux electronics was conducted in the present study. Performance of the copper-finned microstructures were studied as a function of operating parametric values of fin density, fin height, fin length, and channel width over a surface which was rosin soldered to a 10 mm × 10 mm heating block (typical size of an electronic chip). The performance of the copper-finned microstructures versus a flat/smooth nichrome plate in HFE-7100 was significantly higher. Two experimental conditions were investigated. In the first set of experiments pool boiling over the groves was examined, where as in the second set of experiments the fluid was forced-fed into the grooves in a forced convection mode. It is shown that the forced fed mode yields higher heat transfer coefficients than the submerged/pool boiling mode. In general the micro-grooved surfaces performed at least three times better than the flat/smooth surface and preliminary results with the forced-fed evaporation experiments suggest that an order of magnitude heat transfer coefficients are possible when compared with a smooth surface.Copyright

High Performance Micro-Grooved Evaporative Heat Transfer Surface for Low Grade Waste Heat Recovery Applications

The continued demand for high performance electronic products and the simultaneous trend of miniaturization has raised the dissipated power and power densities to new unprecedented levels in electronic systems. Thermal management is becoming increasingly critical to the electronics industry to satisfy the increasing market demand for faster, smaller, lighter and more cost effective products. Utilization of waste heat for the purpose of cooling chip is a promising area for enhancing the thermal management and net energy efficiency of the system. This paper focuses on the development of a tubular microgrooved evaporator and its performance characterization based on heat transfer coefficients and pressure drop measurements. Channel with aspect ratio of 3:1 (channel width – 100 μm, channel height – 300 μm) microgrooved structure was used in the evaporator. The system has been tested with R134a as refrigerant for refrigerant flow rate range of 0.005–0.02 kg/s and water flow rate range of 0.25–0.65 kg/s. Very promising results has been obtained in preliminary investigation. Heat transfer coefficient as high as 13,500 W/m2k has been obtained which is almost five times higher than comparative state of art technologies. The associated pressure drop is quite modest and much less than state of the art conventional evaporators.Copyright

A Experimental Study of Condensation Heat Transfer and Pressure Drop in a Single High Aspect Ratio Micro-Channel for Refrigerant R134a

Only recently, experimental data is available in open literature in condensation of various refrigerants in small hydraulic diameter microchannels. The phenomenon of two-phase flow and heat transfer mechanism in small diameter microchannels (< 1 mm) may be different than that in conventional tube sizes due to increasing dominance of several influencing parameters like surface tension, viscosity etc. This paper presents an on-going experimental study of condensation heat transfer and pressure drop of refrigerant R134a is a single high aspect ratio rectangular microchannel of hydraulic diameter 0.7 mm and aspect ratio 7:1. This data will help explore the condensation phenomenon in microchannels that is necessary in the design and development of small-scale heat exchangers and other compact cooling systems. The inlet vapor qualities between 20% and 80% and mass fluxes of 130 and 200 kg/m2 s have been studied at present. The microchannel outlet conditions are maintained at close to thermodynamic saturated liquid state through a careful experimental procedure. A unique process for fabrication of the microchannel involving milling and electroplating steps has been adopted to maintain the channel geometry close to design values. Measurement instruments are well-calibrated to ensure low system energy balance error, uncertainty and good repeatability of test data. The trends of data recorded are comparable to that found in recent literature on similar dimension tubes.Copyright

Analysis of Taylor Flow in Microchannels by the Phase Field Method

The present paper reports a comprehensive study on the numerical simulation of Taylor flow in microchannels by the phase field method. Additionally, a comparative study was also performed against an alternative volume of fluid model based on which the phase field method was found to be more advantageous in key aspects such as the absence of unphysical interfacial pressure oscillations and the ability to account for variations in the surface tension force and thus predict several bubble lengths under constant flow conditions while observing the physics of homogeneous two-phase flow. Different bubble formation mechanisms were simulated and compared against experimental findings in literature. The simulation of a thin liquid film at the channel wall was found to be limitation of most works pertaining to Taylor flow, including the present. This was ascribed to be more likely due to limited dimensional and spatial resolution as well as inaccurate contact angle dynamics rather than limitations of the modeling approach itself. The effect of wall adhesion was studied with respect to the flow and pressure field in the channel. A validation of the model was achieved through a favorable comparison of the numerically predicted gas void fraction and bubble lengths with existing models and correlations. On the whole, the phase field method was concluded to have improved predictive accuracy with respect to certain aspects as compared to conventional multiphase flow models.Copyright

Numerical Simulation of Mass Transfer Characteristics in a Taylor Flow Microreactor

The aim of this work is to investigate numerically the mass transfer characteristics in a Taylor flow microchannel reactor. Previous attempts to model gas-liquid mass transfer in microchannels have mainly been done by the unit cell based models. Limitations of this approach are its incapability to account for the mass transfer in the inlet mixing region and the dependence on empirical data to define the unit cell geometry. The present work attempts to overcome both these shortcomings by adopting a purely numerical approach to model the mass transfer in a Taylor flow microreactor. A finite-element implementation of the phase field method was used to predict the hydrodynamics of the two-phase flow The flow pattern obtained was used to define the computational domain to model the mass transfer. The reaction system of CO2 absorption into aqueous NaOH solution was considered for gas superficial velocities ranging from 0.09 to 0.25 m/s with the liquid phase superficial velocities ranging from 0.02 to 0.21 m/s. Channels with hydraulic diameters ranging from 100 μm to 500 μm were considered with flow focusing and cross flow types of inlet configuration. The effect of channel length was also studied by varying the residence time in the transient simulation. Results suggest that the conventional unit cell based approaches which do not model the inlet mixing region could over predict the mass transfer by up to 16%. Smaller diameter channels were found to have improved mass transfer characteristics. This was found to be further enhanced by higher concentration levels of the liquid reactant and higher temperatures. The channel wall wettability was found to negligibly affect the mass transfer characteristics. The predictions from the present model were compared with experimental data as well as with predictions of the unit cell based model and a good agreement was obtained with both models.© 2011 ASME

Studies on Condensation of Refrigerants in a High Aspect Ratio Microchannel and in a Novel Micro-Groove Surface Heat Exchanger: Development of Micro-Condensers in Compact Two Phase Cooling Systems

Three different refrigerants, R134a, R245fa and HFE7100 were analyzed as working fluids for two-phase cooling of high heat flux electronics in a 0.7 mm hydraulic diameter 190 mm long high aspect ratio minichannel and in a newly developed micro-groove surface condenser. The latter comprised of a micro-groove surface with rectangular grooves of 84 μm in hydraulic diameter with an aspect ratio of 10.6 and headers that directed the refrigerant flow into the grooves. It was concluded that in the minichannel R245fa provides higher heat transfer coefficients compared to R134a with a significantly higher pressure drop. The saturation temperature drop in the same channel created a significant temperature drop for HFE7100 that make the application of such minichannels for cross-flow condensers with this fluid unpractical. The microgroove surface condenser provided significantly higher heat transfer coefficients compared to the minichannel condenser. The pressure drop in the micro-groove surface condenser was extremely low and imposed just 1C temperature drop on HFE7100 at it highest heat flux. The mass flux of refrigerant in the micro-groove surface condenser is significantly lower compared to conventional mini and microchannel condensers. In its current configuration, the microgroove surface condenser benefits from the possibility of an increase in mass flux resulting in a significant increase in heat transfer coefficient and just a moderate increase in pressure drop.Copyright

Phase Field Method for Simulation of Multiphase Flow

The present paper reports a comprehensive study on the numerical simulation of Taylor flow in microchannels by the phase field method. Additionally, a comparative study was also performed against an alternative volume of fluid model based on which the phase field method was found to be more advantageous in key aspects such as the absence of unphysical interfacial pressure oscillations and the ability to account for variations in the surface tension force and thus predict several bubble lengths under constant flow conditions while observing the physics of homogeneous two-phase flow. Different bubble formation mechanisms were simulated and compared against experimental findings in literature. The simulation of a thin liquid film at the channel wall was found to be a limitation of most works pertaining to Taylor flow, including the present. This was ascribed to be more likely due to limited dimensional and spatial resolution as well as inaccurate contact angle dynamics rather than limitations of the modeling approach itself. The effect of wall adhesion was studied with respect to the flow and pressure field in the channel. A validation of the model was achieved through a favorable comparison of the numerically predicted gas void fraction and bubble lengths with existing models and correlations. On the whole, the phase field method was concluded to have improved predictive accuracy with respect to certain aspects as compared to conventional multiphase flow models.© 2011 ASME

Forced Convection Boiling in Microchannels for Improved Heat Transfer

This paper presents experimental results from research investigating the heat transfer capabilities of microchannel surfaces using a novel force-fed boiling and evaporation technique. The evaporative surfaces being investigated consist of a series of parallel, high-aspect ratio, open topped microchannels. The different sample surfaces vary in channel density, channel aspect ratio, and channel width and have heat transfer surface areas up to ten times their nominal surface areas. Liquid enters the channels of the evaporative surface from above through a developed system of feed channels. This method organizes a liquid-vapor circulation at the boiling surface that results in dissipation of very high heat fluxes in the boiling/thin film evaporation mode. By using the force-fed boiling technique, nominal area heat transfer rates of 100,000 W/m2 -K have been achieved with HFE-7100 as the working fluid [1]. In force-fed boiling, the many very short microchannels are working in parallel; therefore the feed pressure and pumping power are very low. This technique may prove valuable to a wide range of heat transfer applications, particularly for heat removal at high heat flux surfaces.Copyright