Arlington, TX, United States
Arlington, TX, United States

Time filter

Source Type

Ayub Z.H.,Isotherm Inc. | Abbas A.,Ghulam Ishaq Khan Institute of Engineering Sciences and Technology | Ayub A.H.,Isotherm Inc. | Khan T.S.,The Petroleum Institute | Chattha J.A.,Ghulam Ishaq Khan Institute of Engineering Sciences and Technology
International Journal of Refrigeration | Year: 2017

There are no data available on the direct expansion evaporation of refrigerant on the outside of a tube bundle. With the current ozone depletion and global warming issues it is critical to develop systems with low charge refrigerants especially with natural refrigerants such as ammonia which has zero ozone depletion potential (ODP) and zero global warming potential (GWP). This study presents results of an extensive experimental work on direct expansion of ammonia on a triangular pitch plain tube bundle with saturation temperature range −1.7 to −20 °C, heat flux range 5 to 45 kW m−2 and exit superheat range 2 to 10 °C. The test matrix falls well within the practical operating conditions of industrial refrigeration systems. A typical increase in heat transfer coefficient was observed with saturation temperature and heat flux. The effect of exit super heat on the overall performance of the bundle was also reported. A correlation for outside boiling on a tube bundle in direct expansion mode was developed and compared with existing single tube pool, flooded bundle boiling and spray evaporation studies. © 2017 Elsevier Ltd and IIR


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.


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.


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.


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.


Ayub Z.,Isotherm Inc.
Heat Transfer Engineering | Year: 2015

Editor's Note: We at the Journal of Heat Transfer Engineering are happy to announce the introduction of a new section entitle “Case Studies.” We are asking authors to contribute their practical experiences in the form of real-world case studies. The goal is to disseminate practical information especially to young engineers who are stepping into the real world. The format of such articles will be a full-length paper or a short brief. It does not have to entail a typical research-type paper format with details about measurement accuracies, references, and so on. The article could have a brief introduction with the problem description, remedy, lessons learned, and recommendations. The review process will be simple and less involved than with typical research papers. The goal is not to discredit a person or a company but to share experiences. We all make mistakes; however, we also find solutions, and it is the solution part that is rewarding and that should be shared for the betterment of the society. I am opening this section with my own experience 6 years ago. I expect a positive response from fellow engineers. HTE Case Studies Editor 2016 Copyright © Taylor & Francis Group, LLC


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.
Applied Thermal Engineering | Year: 2010

Corrugated plate heat exchangers have larger heat transfer surface area and increased turbulence level due to the corrugations. In this study, experimental heat transfer data are obtained for single phase flow (water-to-water) configurations in a commercial plate heat exchanger for symmetric 30°/30°, 60°/60°, and mixed 30°/60° chevron angle plates. Experiments were carried out for Reynolds number ranging from 500 to 2500 and Prandtl number from 3.5 to 6.5. Experimental results show significant effect of chevron angle and Reynolds number on the heat transfer coefficient. Based on the experimental data, a correlation to estimate Nusselt number as a function of Reynolds number, Prandtl number and chevron angle has been proposed. © 2010 Elsevier Ltd. All rights reserved.


Case Studies Editor's Note: We at the Journal of Heat Transfer Engineering (HTE) are happy to announce the introduction of a new section entitled “Case Studies.” The first case study was published in vol. 37(3–4) and the second case was published in vol. 37(7–8). This is the third case study in this series. We are asking authors to contribute their practical experiences in the form of real-world case studies. The goal is to disseminate practical information especially to young engineers who are stepping into the real world. The format of such articles will be a full-length paper or a short brief. It does not have to entail a typical research-type paper format with details about measurement accuracies, references, and so on. The article could have a brief introduction with the problem description, remedy, lessons learned, and recommendations. The review process will be simple and less involved than with typical research papers. The goal is not to discredit a person or a company but to share experiences. We all make mistakes; however, we also find solutions, and it is the solution part that is rewarding and should be shared for the betterment of the society. Zahid Ayub HTE Case Studies Editor 2016 Copyright © Taylor & Francis Group, LLC


Case Studies Editor's Note: We at the Journal of Heat Transfer Engineering (HTE) are happy to announce the introduction of a new section entitled "Case Studies." The first case study was published in vol. 37(3-4) and the second case was published in vol. 37(7-8). This is the third case study in this series. We are asking authors to contribute their practical experiences in the form of real-world case studies. The goal is to disseminate practical information especially to young engineers who are stepping into the real world. The format of such articles will be a full-length paper or a short brief. It does not have to entail a typical research-type paper format with details about measurement accuracies, references, and so on. The article could have a brief introduction with the problem description, remedy, lessons learned, and recommendations. The review process will be simple and less involved than with typical research papers. The goal is not to discredit a person or a company but to share experiences. We all make mistakes; however, we also find solutions, and it is the solution part that is rewarding and should be shared for the betterment of the society.Zahid AyubHTE Case Studies Editor. © 2016 Taylor and Francis Group, LLC.


Hayes N.,Hill Engineering, LLC | Jokar A.,ThermoFluids Technology | Ayub Z.H.,Isotherm Inc.
International Journal of Heat and Mass Transfer | Year: 2011

The experimental investigation of carbon dioxide condensation in brazed plate heat exchangers is the main objective of this study. The current level of concern for the environment is at an all time high, therefore, it is important to look into methods and resources that lead to a cleaner and healthier future for the planet. This study details one such effort to reach this goal, focusing on condensation of carbon dioxide as a natural refrigerant in refrigeration systems. Three brazed plate heat exchangers with different geometry, each consisting of three channels, are tested. This paper focuses on the two-phase analysis, where carbon dioxide was the working fluid, flowing through the middle channel, and dynalene brine, the cooling fluid, flowed through the side channels of each geometry. Condensation of carbon dioxide occurred at saturation temperatures ranging from -17.8 °C to -34.4 °C and heat fluxes spanning 2.5-15.7 kW/m2. An in-depth dimensional analysis was completed on the two-phase data yielding heat transfer correlations. Relationships of the two-phase heat transfer characteristics are presented, the data are compared with related studies, and conclusions are made from the two-phase data. © 2010 Elsevier Ltd. All rights reserved.

Loading Isotherm Inc. collaborators
Loading Isotherm Inc. collaborators