Ebrahim Al Hajri | The Petroleum Institute (original) (raw)

Papers by Ebrahim Al Hajri

This thesis presents a detailed study of parametric characterization of two-phase condensing flow... more This thesis presents a detailed study of parametric characterization of two-phase condensing flow of two selected refrigerants R134a and R-245fa in a single water-cooled micro-channel of 0.4 mm X 2.8 mm cross-section (0.7 mm hydraulic diameter and 7:1 aspect ratio) and 190 mm in length. To avoid flow mal-distribution associated with typical micro-channel tube banks, a single micro-channel was fabricated utilizing an innovative approach and used for the present study experiments. The study investigated the effects of variations in saturation temperature ranging from 30 o C to 70 o C, mass flux from 50 to 500 kg/m 2 s, and inlet super heat from 0 o C to 15 o C on the average heat transfer and overall pressure drop coefficient of the micro-channel condenser. In all cases the inlet vapor quality was kept at 100% quality (saturated vapor) and the outlet condition was always kept at 0% quality (saturated liquid). Accuracy of the fabricated channel geometry with careful design

Thermal-hydraulic performance of an enhanced tube was evaluated using experimental and numerical ... more Thermal-hydraulic performance of an enhanced tube was evaluated using experimental and numerical simulation techniques in a pipe-in-pipe heat exchanger. Steady state single phase (liquid-to-liquid) experiments were performed to determine Nusselt number and friction factor. Experiments with water as working fluid were carried out in the Reynolds number range of (500 < Re < 8000), while for water/ glycol solution based experiments the Reynolds number range was kept at (150 < Re < 2000). A nondimensional performance evaluation criterion (PEC) was used to assess the thermal-hydraulic performance of heat transfer enhancement achieved with the enhanced tube. Based on the experimental data, Nusselt number and friction factor estimation correlations were proposed for the enhanced tube.

Manifold-microchannel technology has demonstrated substantial promise for superior performance ov... more Manifold-microchannel technology has demonstrated substantial promise for superior performance over state of the art heat exchangers, with potential to reduce pressure drop considerably while maintaining the same or higher heat transfer capacity compared to conventional microchannel designs. However, optimum design of heat exchangers based on this technology requires careful selection of several critical geometrical and flow parameters. The present research focuses on the numerical modeling and optimization of a manifold-microchannel plate heat exchanger to determine the design parameters that yield the optimum performance for the heat exchanger. A hybrid method that requires significantly shorter computational time than the full Computational Fluid Dynamic (CFD) model was developed to calculate the coefficient of performance and heat transfer rates of the heat exchanger. The results from the hybrid method were successfully verified with the results obtained from a full CFD simulation and experimental work. A corresponding multi-objective optimization of the heat exchanger was conducted utilizing an approximation-based optimization technique. The optimized manifold-microchannel plate heat exchanger showed superior heat transfer performance over chevron plate heat exchanger designs.

Effective heat and mass exchangers are vital for further improvement of absorption cooling system... more 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-aspectratio 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/m 2 -K was measured with a modest maximum pressure drop of 120 mbars and 100 mbars on the refrigerant and water sides, respectively.

This paper reports the results of a finite element implementation of the phase field method for t... more This paper reports the results of a finite element implementation of the phase field method for the simulation of Taylor flow in mini/microchannels. Certain characteristics of Taylor flow have been frequently investigated in the literature using conventional modeling approaches but are relatively less frequently investigated using the phase field formulation, the focus of the present study. Modeling of wall adhesion has been studied by means of a simpler spinodal decomposition problem that isolates the phase field equations from those used to govern the flow hydrodynamics. Next, studies on twophase flow are based on the phase distribution, flow field and pressure distribution in the computational domain. The current predictions are compared against select simulations performed with the volume of fluid model. The ability of both models to capture the thin liquid film between the bubble and channel wall is assessed and directions for future work are provided. The absence of unphysical parasitic velocity currents from the phase field results is demonstrated. The pressure distribution was characterized by the absence of interfacial oscillations in pressure that were shown to be present with the volume of fluid model. These can be considered major advantages of the phase field model over alternative approaches. Lastly, the effect of channel inlet configuration is studied and discussed.

Multiphase heat and mass transfer in microscale devices is a growing field of research due to the... more Multiphase heat and mass transfer in microscale devices is a growing field of research due to the potential of these devices for use in various engineering applications. Before the heat and mass transport phenomena in such systems can be modeled, the hydrodynamics of adiabatic multiphase flow, in the absence of specie transport across interfaces, must be accurately predicted. In the present paper, a finite element implementation of the phase field method is applied to simulate Taylor flow in mini/microchannels. Channels with characteristic dimensions ranging from 100 to 500 mm are modeled and criteria present in the literature for domain discretization are assessed. The effects of phase field parameters, namely mobility and interface thickness, on the predicted flow features are discussed. The predicted Taylor bubble lengths are compared against empirical correlations as well as available experimental data in the literature. The predicted gas void fraction data for different channel dimensions are compared with numerous experimental studies. The present results indicate a linear variation of gas void fraction with respect to volumetric flow ratio for all channel sizes.

Heat transfer coefficient Pressure drop Experimentation Correlations a b s t r a c t An experimen... more Heat transfer coefficient Pressure drop Experimentation Correlations a b s t r a c t An experimental study on parametric characterization of two-phase condensing flows of refrigerants R134a and R245fa in a single microchannel was carried out utilizing a microchannel with a cross-section of 0.4 mm Â 2.8 mm (7:1 aspect ratio) and length of 190 mm.

Numerical simulation of mass transfer in Taylor flow microchannel reactors. Purely theoretical ap... more Numerical simulation of mass transfer in Taylor flow microchannel reactors. Purely theoretical approach that models channel length and inlet mixing region. Reaction system of absorption of carbon dioxide in aqueous sodium hydroxide. Effect of length, wettability, inlet mixing, phase concentrations, temperature. Predicted results compare favorably with experimental data in the literature.

Performance enhancement of heat exchangers with a focus in optimum weight/volume and the amount o... more Performance enhancement of heat exchangers with a focus in optimum weight/volume and the amount of working fluid in circulation is of significance to a diverse range of industries. This paper presents heat transfer and pressure drop characteristics of a compact tubular evaporator which utilizes a manifold force-fed microchannel design. A microgrooved structure with an aspect ratio of 3:1 (channel width of 100 lm and channel height of 300 lm) forms the channels used on the refrigerant side and minichannels of 1 mm depth were used on the water side. The system was tested using R134a as the refrigerant with a refrigerant flow rate of 6 to 22 g/s and water flow rate of 150 to 640 ml/s. Overall heat transfer coefficients of more than 10,000 W/m 2 K were obtained with modest values of pressure drop. The present results indicate a significant enhancement in thermal performance when compared to the state-of-the-art technologies in the same application area.

In today's fast growing world where availability of energy has become a major concern, the cost o... more In today's fast growing world where availability of energy has become a major concern, the cost of performance demands optimum heat exchange performance over extended periods of operational times. Fouling is one major factor that drastically affects heat exchanger performance. Most of the oil & gas processing plants in the Middle East are located in deserts. Due to scarcity of water most of the installed heat exchangers are air-cooled. These heat exchangers are at high risk of low performance due to dusty/sticky particulate fouling. In order to identify possible active/passive methods to control or ideally eliminate particulate fouling, as a first step, it is desirable to know exact morphology of such particulate fouling. This study presents morphological characterization of selected fouling samples from eight different installed fin fan heat exchangers. The scanning electron microscope (SEM) tests are carried out to determine standard characteristics and size of sample foulant powder. Variability in sizes and shapes is found between samples perhaps due to different working temperature ranges of the selected heat exchangers. The semi quantitative sample composition measured by energy dispersion x-ray micro analysis was as following: 26.50% Si, 26.12% Ca, 10.07% C and 9% Al with traces of Fe, Na, Mg, Cl, and some other salts. X-ray diffraction analysis revealed presence of quartz, calcite and alumina with traces of halite and hematite. The diversity of these fouling samples reflects complexity with respect to their potential removal and effects on heat transfer.

The quest to achieve higher heat transfer rate, smaller size and minimum pressure drop is a main ... more The quest to achieve higher heat transfer rate, smaller size and minimum pressure drop is a main area of focus in the design of heat exchangers. Plate heat exchangers are one of viable candidates to deliver higher heat duties but still have a drawback of higher pressure drop due to long restricted flow path. Motivated by demand of miniaturization and cost reduction, a novel design of tubular microchannel heat exchanger for single phase flow employing ammonia water mixture is proposed. Numerical simulation of unit fluid domain is conducted in ANSYS Fluent. Parametric study of the different flow geometries is evaluated in terms of Nusselt number and pressure drop. The salient features of the design include ultra-compact size with higher heat transfer rate and acceptable pressure drop.

Heat exchangers are widely used in industry. Heat transfer enhancement can play vital role in ene... more Heat exchangers are widely used in industry. Heat transfer enhancement can play vital role in energy conservation and space saving. In the current study heat transfer and pressure drop characteristics of a commercial enhanced tube having dimples on the surface were studied using experimental and numerical methods. A double pipe heat exchanger was built to study single phase (liquid to liquid) heat transfer and pressure drop for both smooth and enhanced tubes. The heat exchanger was configured in counter flow arrangement. For experiments, the Reynolds number was varied from 500 to about 8,000. Modified Wilson plot method was used to obtain the convective heat transfer coefficient in the inner tube that contained hot fluid. Experimental results show considerable effects of surface enhancement and Reynolds number on heat transfer performance. The heat transfer augmentation achieved is more than the friction factor penalty. Simulations were conducted to find the heat transfer performance and pressure drop penalty of dimpled tube. Commercial CFD software Fluent was used to obtain heat transfer coefficient and pressure drop. The realizable k-ԑ model was employed to predict the influence of dimples on heat transfer enhancement in turbulent flow. Simulation results showed considerable increase in heat transfer for dimpled tube compared to smooth tube. The simulation results showed good agreement with experimental results. Non dimensional performance evaluation criteria show promising results for enhanced tube.

This paper describes a multi-objective optimization of single-phase, laminar flow inside a single... more This paper describes a multi-objective optimization of single-phase, laminar flow inside a single element of a manifold microchannel flat plate heat exchanger. Approximation assisted optimization was used for the optimization process. The process uses metamodeling in conjunction with Computational Fluid Dynamic (CFD) simulation as a method to minimize the number of function evaluations and thereby obtain substantial reductions in computational time. Two optimization objectives were considered: a) maximizing heat density rate per temperature difference Q/(VΔT) and minimizing pumping power density (P/V), and b) maximizing base heat transfer coefficient (h) and minimizing pumping power per base area (P/A base ). Water and air were used as working fluids to compare the optimum solutions of the two fluids with very distinctive thermophysical properties. The study shows that both optimization objectives result in similar optimum points. The behaviors of the optimum solutions for water and air are also discussed in detail. Additionally, as a case study using the optimization results, it was demonstrated that for an array of microchannels with volume as low as 4,250 mm 3 on one side, pumping power of 138 W and heat transfer rate of 56.7 kW can be achieved using water.

The UAE Government is actively attempting to lead the efforts of reducing CO2 emissions by encour... more The UAE Government is actively attempting to lead the efforts of reducing CO2 emissions by encouraging the process of recycling materials and implementing renewable energy sources. Currently, the collection of recyclable material is via different labeled bins which hold the materials. Even though such configuration is a very cost effective, people tend not to follow instructions hence requiring the mixed materials inside the bin to be sorted after collection. Therefore, this paper proposes a system for autonomously identifying, handling, and sorting plastic, aluminum, and glass waste products. The system is intended to operate in an outdoor environment and therefore, solar energy technology is integrated as its primary power source. Many challenges are present in the system particularly in reliably identifying the various materials. Hence, an innovative material identification method were tested and implemented in the system. The method introduces the principle of recording, analyzing and identifying the recyclable materials from the sound effect generated when the object is dropped on a hard surface. The system was tested by throwing 300 samples of plastic, glass, aluminum with different shapes, sizes, and colors in both quiet and noisy environments. Out of 300 samples, only 1 sample was identified incorrectly which gives a success rate of 99.67%. Therefore, this system is an effective solution to reduce the cost and time required by material recovery facilities in the recycling process. In order to encourage the community to recycle, the system is enhanced by a novel idea where money is given in exchange for the recyclable objects based on weight and material through coding and limits based on the UAE Standards. The user has also the option of donating the accumulated sum for a specific charity.

Carbon dioxide (CO 2 ) is the largest volume contributor and the fastest growing component of gre... more Carbon dioxide (CO 2 ) is the largest volume contributor and the fastest growing component of greenhouse gases. Based on current technology the only commercially available process that can absorb a reasonable amount of CO 2 from flue gases is chemical absorption. The other techniques are generally less energy efficient and more expensive. Microchannel technology can be used to enhance the mass transfer rate by increasing surface-to-volume ratio and improving the thermal controllability of the absorption process. In the current study we investigated the performance of microchannel contactors for absorption of CO 2 in aqueous diethanolamine (DEA). A series of experiments was performed to measure CO 2 absorption rate and removal efficiency for various gas-to-amine flow rate ratios. The rate of absorption was determined based on the variation of electrical conductivity of the aqueous DEA due to the CO 2 absorption process. The effect of contactor length was studied for 200, 500, and 800 mm long microchannels. The pressure drops of two-phase flow for various flow rate ratios and microchannel length were measured. The results demonstrated high potential of the microchannel contactors for enhancement of the absorption process.