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Li M.,The Petroleum Institute | Khan T.S.,The Petroleum Institute | Al Hajri E.,The Petroleum Institute | Ayub Z.H.,Isotherm Inc.
Applied Thermal Engineering | Year: 2016

Enhanced surfaces have larger heat transfer surface area and offer increased turbulence level hence allowing higher heat exchange performance. In this study, numerical simulations are conducted to simulate geometric design optimization of enhanced tubes for optimal thermal-hydraulic performance. The simulations are validated with experimental data. Two and three dimensional steady incompressible turbulent flow in dimpled enhanced tube is numerically studied using realizable k-ϵ method. The pressure-velocity coupling is solved by Semi-Implicit Method for Pressure Linked Equations Consistent (SIMPLEC) algorithm. Results show that dimples on tube surface present high heat transfer performance. Compared to staggered configuration, the in-line dimples arrangement provided better overall heat exchange characteristics. The geometric parameters like dimple shape, depth, pitch and starts are found to have significant effects on overall heat exchange performance while the dimple diameter has insignificant effect on thermal performance. © 2016 Published by Elsevier Ltd. Source


Khan M.S.,Mohammad Ali Jinnah University | Khan T.S.,Ghulam Ishaq Khan Institute of Engineering Sciences and Technology | Chyu M.-C.,Texas Tech University | Ayub Z.H.,Isotherm Inc.
International Journal of Refrigeration | Year: 2012

Ammonia, an environment friendly natural refrigerant with attractive thermo-physical properties is a potential replacement of the synthetic refrigerants with high ozone depletion and global warming potentials. Experimental heat transfer and pressure drop data are obtained for two phase evaporation of ammonia in a commercial plate heat exchanger for symmetric 30°/30° chevron plates. The plate heat exchanger was configured in a single pass U-type counter flow arrangement with liquid ammonia evaporating in upward flow direction and glycol/water solution simulating the heat load in counter flow. Experiments were carried out for saturation temperatures ranging from -25 °C to -2 °C. The heat flux was varied between 21 kW m -2 and 44 kW m -2. Experimental results show significant effect of saturation temperature, heat and exit vapor quality on heat transfer coefficient and pressure drop. Two phase Nusselt number and friction factor correlations are proposed. © 2012 Elsevier Ltd and IIR. All rights reserved. Source


Khan T.S.,Ghulam Ishaq Khan Institute of Engineering Sciences and Technology | Khan M.S.,Ghulam Ishaq Khan Institute of Engineering Sciences and Technology | Chyu M.-C.,Texas Tech University | Ayub Z.H.,Isotherm Inc.
International Journal of Refrigeration | Year: 2012

Ammonia is an environment friendly natural refrigerant with attractive thermo-physical properties. Experimental heat transfer and pressure drop data are obtained for evaporation of liquid ammonia in a commercial plate heat exchanger for symmetric 60°/60° (hard) chevron angle plates. Experiments were carried out for mass flux ranging from 8.5 to 27 kg m -2 s -1 at saturation temperatures ranging from -25 °C to -2 °C. The heat flux was varied between 21 kW m -2 and 44 kW m -2. The heat transfer coefficient increased with an increase in saturation temperature and mass flux. Furthermore, heat transfer coefficient was observed to increase with exit vapor quality. The friction factor decreased with exit vapor quality and equivalent Reynolds number, while it increased with the fluid temperature. The work reveals that ammonia has far better heat transfer and pressure drop characteristics compared to HFCs. Two phase Nusselt number and friction factor correlations are also proposed. © 2011 Elsevier Ltd and IIR. Source


Li M.,The Petroleum Institute | Khan T.S.,The Petroleum Institute | Al-Hajri E.,The Petroleum Institute | Ayub Z.H.,Isotherm Inc.
Applied Thermal Engineering | Year: 2016

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 non-dimensional 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. Simulations were carried out to obtain heat transfer and pressure drop characteristics of smooth and enhanced tubes, using commercial CFD software, Fluent. Realizable k-ϵ model was employed to assess the influence of dimples on turbulent flow and velocity field. Simulation results showed that dimples disturb and mix boundary layers and generate secondary flows that improve turbulence level. Comparison between experimental and numerical simulation results showed good agreement as the data fell within ±10% error band. © 2016 Elsevier Ltd. All rights reserved. Source


Hayes N.,Hill Engineering, LLC | Jokar A.,ThermoFluids Technology | Ayub Z.H.,Isotherm Inc.
International Journal of Heat and Mass Transfer | Year: 2012

Condensation pressure drop of carbon dioxide in brazed plate heat exchangers was investigated, and is presented in this paper. Carbon dioxide is known as an environmental friendly refrigerant with an Ozone Depletion Potential (ODP) equal to zero and Global Warming Potential (GWP) equal to unity, and has favorable thermodynamic and transport properties though it requires higher operating pressures (∼15-30 bar). Brazed-type plate heat exchangers that can withstand high pressure are a good choice for such applications. This paper presents the procedure, data collection, and results for three brazed plate heat exchangers with different inner geometries. The test exchangers showed good performance at high system pressures with reasonable pressure drops (less than 8%). The collected experimental data that covered real world operating conditions are valuable for the design of cascade condensers with carbon dioxide as the low-side refrigerant. © 2012 Elsevier Ltd. All rights reserved. Source

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