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Counterflow logarithmic mean temperature difference is actually the upper bound: A demonstration

APPLIED THERMAL ENGINEERING 2011年第6-7期31卷 1172-1175页

作者： Cartaxo, Samuel J. M. Fernandes, Fabiano A. N. Univ Fed Ceara Dept Engn Quim BR-60455760 Fortaleza Ceara Brazil

The logarithmic mean temperature difference (Delta T-LM or LMTD) is a corner stone concept used in the design and analysis of heat transfer equipment, such as heat exchangers. The LMTD methods are well established in the Chemical Engineering framework and, in general, apply a counterflow LMTD corrected by some factor to assess the "true" mean temperature difference between the fluids exchanging heat. The counterflow LMTD is used because it is undoubtedly believed that its value for two countercurrent streams is higher than when the same streams are arranged in concurrent flow. Although this principle may be supported by direct comparison between the counterflow and parallel flow LMTD's for any given process conditions, a demonstration of its universal validity is not found in the literature. This article presents a proof founded in mathematical and physical arguments that this belief is indeed a fact. (C) 2010 Elsevier Ltd. All rights reserved.

关键词： Heat exchanger logarithmic mean temperature difference Heat transfer Transport processes Design Unit operations

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Prediction of local heat transfer characteristics of dilute gas-solid flows through an adiabatic, horizontal pipe

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HEAT TRANSFER-ASIAN RESEARCH 2019年第6期48卷 1987-2006页

作者： Patro, Brundaban Kumar, K. Kiran Krishna, D. Jaya Natl Inst Technol Dept Mech Engn Warangal 506004 Telangana India

The local heat transfer characteristics of gas-solid flows through an adiabatic, horizontal pipe are numerically studied using the two-fluid model of Ansys Fluent 15. First, the model is validated with the experimental results available in the literature for the air temperature and average Nusselt number. Then, the local heat transfer characteristics of gas-solid flows, such as temperature profiles of gas and solid, gas-solid Nusselt number, logarithmic mean temperature difference, and effectiveness of gas and solid, are studied by changing different parameters (gas velocities 15-24 m/s;inlet solid loading ratios 0.1-1;particle diameters 100-400 mu m). It is observed that increasing the particle diameter and inlet gas velocity increases the gas temperature and decreases the solid temperature, increases the logarithmic mean temperature difference, and decreases the thermal effectiveness of gas and solid. However, increasing the solid loading ratio decreases the gas and solid temperatures, decreases the logarithmic mean temperature difference, and increases the thermal effectiveness of gas and decreases the thermal effectiveness of solid. Moreover, increasing the particle diameter decreases the gas-solid Nusselt number, whereas increasing the solid loading ratio and inlet gas velocity increase the gas-solid Nusselt number.

关键词： dilute phase gas-solid flow local heat transfer logarithmic mean temperature difference thermal effectiveness two-fluid model

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Enhancement of Gas Pumping and Heat Transfer Using a Two-Stage Electrohydrodynamic Pump Operated at Uneven Applied Voltage

Enhancement of Gas Pumping and Heat Transfer Using a Two-Sta...

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AIAA Aviation Forum and Exposition

作者： Mazumder, A. K. M. M. H. Lail, F. C. Saginaw Valley State Univ Dept Mech Engn University Ctr PA 48710 USA Univ Oklahoma Sch Aerosp & Mech Engn 865 Asp Ave Norman OK 73019 USA

ISBN: (数字)9781624105890

ISBN: (纸本)9781624105890

A two-stage electrohydrodynamic (EHD) gas pump inside a square channel is numerically examined for its potential in the enhancement of gas pumping and heat transfer. The gas pump with 28 emitting electrodes in each stage is charged at a combination of three different operating voltages (20 kV, 24 kV, and 28 kV) for the possible performance improvement. The emitting electrodes are flush mounted on the channel walls so that the corona wind produced can directly disturb the development of momentum and thermal boundary layers to create maximum enhancement in gas pumping and heat transfer. The pumping power required for the heat transfer enhancement is also critically evaluated. The enhancement produced by the EHD gas pump is found to be higher than other techniques and with a smaller power penalty. The overall effectiveness of the EHD gas pump in the enhancement is evaluated using the thermal hydraulic parameter, (Nu/Nu(0))/(f/f(0)), which is defined as the ratio of heat transfer enhancement over the power penalty. It is found that this parameter is always greater than unity for the EHD pump considered. These results reveal that EHD gas pump has a great potential for applications in thermal management and can be more energy-efficient when operated with uneven applied voltages.

关键词： logarithmic mean temperature difference Momentum Boundary Layer Electrohydrodynamic Pump Thermal Management temperature Fields Friction Factor Incompressible Flow Electric Field Strength Natural Convection Corona Discharge

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Enhancement of Forced Convection Using a Two-Stage Electrohydrodynamic Gas Pump

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JOURNAL OF THERMOPHYSICS AND HEAT TRANSFER 2018年第3期32卷 617-626页

作者： Mazumder, A. K. M. Monayem H. Lai, Feng C. Lamar Univ Dept Mech Engn 4400 MLK Blvd Beaumont TX 77710 USA Univ Oklahoma Sch Aerosp & Mech Engn 865 Asp Ave Norman OK 73019 USA

A two-stage electrohydrodynamic (EHD) gas pump is numerically examined for its potential to enhance forced convection in a square channel. The EHD gas pump with 28 emitting electrodes in each stage is evaluated for a wide range of operating voltages starting from 20kV up to 28kV. The influence of electric field on the flow and temperature fields are also examined for a wide range of Reynolds numbers varying between 100 and 2000. The emitting electrodes of the EHD gas pump are flush mounted on the channel walls so that the corona wind produced can directly disturb the boundary layers to create the maximum enhancement. The pumping power required for the heat transfer enhancement is also critically evaluated. The maximum increase in the average Nusselt number is 30%, which is achieved at Reynolds number Re=200 with an applied voltage of 28kV. The overall effectiveness of the EHD gas pump in heat transfer enhancement is evaluated by the parameter of thermal hydraulic performance, (Nu/Nu(0))/(f/f(0)), which is found to be always greater than unity. These results reveal that EHD gas pump has a great potential for applications in thermal management.

关键词： Natural Convection logarithmic mean temperature difference Reynolds Numbers Momentum Boundary Layer temperature Fields Thermal Management Electrohydrodynamic Gas Pump Friction Factor Electric Field Strength Numerical Simulation

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Experimental Investigation of Metal Oxide Nanofluids in a Plate Heat Exchanger

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JOURNAL OF THERMOPHYSICS AND HEAT TRANSFER 2019年第4期33卷 994-1005页

作者： Meisam, Ansarpour Ahmad, Azari Hamed, Mohammaddoost Persian Gulf Univ Fac Petr Gas & Petrochem Engn POB 75169-13817 Bushehr 75169 Iran Persian Gulf Univ Oil & Gas Res Ctr Bushehr 75169 Iran

In this work, the experimental heat transfer of metal oxide nanofluids in a plate heat exchanger was investigated. For this purpose, Al2O3 (20and40nm)/water, TiO2 (10-15and25nm)/water, and SiO2 (20-30nm)/water nanofluids with 0.05, 0.1, and 0.2 wt % of nanoparticle concentration were used to investigate the increment of the heat transfer coefficient for each nanofluid against distilled water. Nanofluid characterization was conducted by dynamic light scattering analysis. Significant enhancements on the heat transfer coefficient in the present nanofluids were observed. The results clearly show that increasing the nanofluid Reynolds number and concentration results in an increase in their heat transfer coefficient. The maximum observed heat transfer enhancement for SiO2 was obtained at 0.2 wt % and Reynoldsnumber=37, which is 2.82. Furthermore, the minimum heat transfer enhancement for Al2O3 (20 nm) was achieved at 0.1 wt % and Reynoldsnumber=158 with 1.64. A new correlation was derived for Nusselt number based on the experimental data of the mentioned nanofluids with an average relative error percentage of 3.78%. Moreover, the variations of Nusselt number vs Peclet number, the overall heat transfer coefficient vs Reynolds number, and the nanofluid friction factor vs Reynolds number were considered, and interesting results were reported.

关键词： Plate Heat Exchanger Nanofluids logarithmic mean temperature difference Reynolds Number Natural Convection Friction Factor Dynamic Light Scattering Nanomaterials Specific Heat Capacity Flow Conditions

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Optimization of Crossflow Staggered Tube Banks for Prescribed Pressure Loss and Effectiveness

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JOURNAL OF THERMOPHYSICS AND HEAT TRANSFER 2017年第4期31卷 878-888页

作者： Yilmaz, Alper Erdinc, Mehmet Tahir Yilmaz, Tuncay Cukurova Univ Dept Automot Engn TR-01330 Adana Turkey Osmaniye Korkut Ata Univ Dept Mech Engn TR-80000 Osmaniye Turkey

For many applications, it is important to transfer a given amount of heat with minimum possible volume of heat exchanger for a given pressure loss and effectiveness. In this study, optimum design of crossflow staggered tube banks for prescribed pressure loss and effectiveness is carried out. For this purpose, the method of Gnielinski-Gaddis is used for the calculation of heat transfer coefficient and pressure loss in tube banks. By prescribing the tube diameter, heat exchanger effectiveness, and pressure loss of the heat exchanger, the pitch length and tube number in the flow direction are determined for the condition of the maximum heat flow per heat exchanger volume. For a given value of total heat to be transferred, the length, height, and width of the heat exchanger can be determined by the equations that are derived using appropriate dimensionless numbers.

关键词： logarithmic mean temperature difference Heat Exchangers Flow Conditions Finite Volume Method Multi Objective Genetic Algorithm Mass Flow Rate Specific Heat Capacity Inlet temperature Wall temperature Natural Convection

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Optimal Cruise Altitude for Aircraft Thermal Management

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JOURNAL OF GUIDANCE CONTROL AND DYNAMICS 2015年第11期38卷 2084-2095页

作者： Doman, David B. US Air Force Res Lab Power & Control Div Aerosp Syst Directorate Wright Patterson AFB OH 45433 USA

Mission-planning algorithms and trajectory optimization methods for aircraft that use fuel as a heat sink must ensure that fuel temperature limits are not violated at any point in a flight. The problem of determining the cruise altitude that minimizes final fuel tank temperature for jet aircraft that use recirculating fuel to remove waste thermal energy from the aircraft is considered. A heat exchanger transfers energy from the aircraft subsystems to the fuel. A portion of the fuel is burned in a turbojet engine, whereas the remaining unburned fuel is passed through a cooler that transfers some heat from the fuel to the atmosphere. A differential equation that describes the time rate of change of fuel tank temperature is derived based on the principles of conservation of mass and energy. Specific conditions are derived for the cruise altitude that minimizes final tank temperature for flight conditions associated with maximum endurance and range. Expressions are also derived that provide estimates of time and range to a temperature limit. Numerical examples are provided to illustrate the utility of the conditions and to assess the impact of the assumptions made in the derivation of the analytic models. The conditions also provide a relationship between aircraft performance parameters, the standard atmosphere, and the heat transfer characteristics of the fuel recirculation loop.

关键词： Cruise Altitude Transonic Flight Thermal Management System logarithmic mean temperature difference Heat Exchangers Thermal Energy Aircraft Performance Jet Aircraft Turbofan Engines Trajectory Optimization

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Heat Transfer of Cu-Water Nanofluid in Parallel, Corrugated, and Strip Channels

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JOURNAL OF THERMOPHYSICS AND HEAT TRANSFER 2015年第4期29卷 747-756页

作者： Khoshvaght-Aliabadi, Morteza Hormozi, Faramarz Islamic Azad Univ Dept Chem Engn Shahrood Branch Shahrood *** Iran Semnan Univ Sch Chem Petr & Gas Engn Semnan *** Iran

Influences of corrugated-wavy and offset-strip channels as the core surface of plate-fin heat exchangers and copper nanoparticles as additives in deionized water were individually investigated. Then, the use of the corrugated-wavy and offset-strip channels along with the Cu-water nanofluid was examined as a compound heat transfer enhancement technique. The nanofluids were prepared by a one-step method, namely, electro-exploded wire technique, and the required properties of the nanofluids were systematically measured. It was found that the corrugated-wavy channel has a drastic effect on the performance of the heat exchanger, and the nanofluid with the lowest weight fraction (i.e., 0.1%) presents a better thermal-hydraulic performance. Moreover, the thermal-hydraulic specification associated with the application of the corrugated-wavy channel along with Cu-water nanofluid was discovered to be higher than those presented by the individual techniques. This compound technique can improve the considered performance factor by an average value of around 1.46 in the plate-fin heat exchanger.

关键词： Nanofluids logarithmic mean temperature difference Nanoparticles Plate Fin Heat Exchanger Specific Heat Capacity Flow Characteristics Boundary Layer Thickness Friction Factor Prandtl Number Fanning Friction Factor

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Wavy Channel and Different Nanofluids Effects on Performance of Plate-Fin Heat Exchangers

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JOURNAL OF THERMOPHYSICS AND HEAT TRANSFER 2014年第3期28卷 474-484页

作者： Khoshvaght-Aliabadi, M. Zamzamian, A. Hormozi, F. Semnan Univ Fac Chem Petr & Gas Engn Semnan *** Iran Mat & Energy Res Ctr Karaj 131451659 Iran

In the present research, an experimental study of forced convective flows of different nanofluids through a corrugated wavy channel at a constant wall temperature condition was performed. Effects of different parameters, including nanoparticle concentration (copper nanoparticles in deionized water), types of oxide nanoparticles (SiO2, TiO2, ZnO, Fe2O3, Al2O3, and CuO), and types of base fluid (deionized water-ethylene glycol mixture), for different volumetric flow rates were examined. The convective heat transfer coefficient and pressure drop measurements were carried out. Results indicate that, for all the values of the flow rate, the Nusselt number augments and Fanning friction factor reduces as the concentration of the copper nanoparticles decreases. In addition, it is shown that the SiO2-deionized water nanofluid yields the best thermal-hydraulic performance among the studied oxide nanoparticles in deionized water, and the TiO2- and CuO-deionized water nanofluids come in second and third. It is also found, whereas the thermal-hydraulic performance improves with increasing the mass of ethylene glycol in the deionized water base fluid, the SiO2 nanoparticles in a base fluid with the lower mass of the ethylene glycol in the deionized water shows a better performance.

关键词： Nanofluids Plate Fin Heat Exchanger Nanomaterials logarithmic mean temperature difference Fanning Friction Factor Thermal Diffusivity Specific Heat Capacity Prandtl Numbers Friction Factor Flow Characteristics

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Modeling a Freezable Water-Based Heat Exchanger for Use in Spacecraft Thermal Control

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JOURNAL OF THERMOPHYSICS AND HEAT TRANSFER 2014年第4期28卷 708-716页

作者： Nabity, James A. Univ Colorado Dept Aerosp Engn Sci Boulder CO 80309 USA

A spacecraft thermal control system typically uses water coolant loops to keep the cabin comfortable. However, the relatively high freeze point of water limits its use, because freezing water within the loop may damage the thermal control system. This paper describes a novel freeze-tolerant heat exchanger and the thermal model developed to correlate data from experiments. The radial transport of heat within the heat exchanger was modeled as a thermal circuit. The heat flows from the water into the thermally conductive shell and fins. One flow channel within the heat exchanger was lined with insulation to keep it free of ice. When the heat sink temperature is below the freeze point of water, a layer of ice builds up in the other flow channels to greatly increase the thermal resistance to heat transport, since the thermal conductivity of ice is only about 1/80th that of Al-6061. In addition, the wetted surface area is reduced by a factor of six when only the insulated channel can still flow water ( all of the other channels are frozen). Together, these two effects turn down the rate of heat rejection in proportion to the net thermal load and the external heat sink environment encountered.

关键词： Spacecraft Thermal Control Heat Exchangers logarithmic mean temperature difference Thermal Resistance Thermal Conductivity Coolant Active Thermal Control Thermal Modeling Space Radiators Chiller

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