Showing papers in "Heat Transfer Engineering in 2014"

Review of the Manufacturing Techniques for Porous Surfaces Used in Enhanced Pool Boiling

Abstract: Continuous development of high-performance microelectronic chips requires efficient cooling systems to dissipate large amount of heat produced over a small footprint. Pool boiling is capable of dissipating large heat fluxes while maintaining low wall superheat and is receiving renewed interest. Porous surfaces have been investigated extensively for pool boiling enhancement. This paper presents a review of different manufacturing techniques employed to manufacture porous surfaces in pool boiling application. Different types of surfaces developed using these techniques are reviewed and their pool boiling performance is discussed.

Condensation in a Square Minichannel: Application of the VOF Method

Abstract: A number of steady-state numerical simulations of condensation of R134a at mass fluxes of 400 kg m−2 s−1 and 800 kg m−2 s−1 inside a 1-mm square cross section minichannel are proposed here and compared against simulations in a circular cross section channel with the same hydraulic diameter. The volume of fluid (VOF) method is used to track the vapor–liquid interface, and the effects of interfacial shear stress, surface tension, and gravity are taken into account. A uniform wall temperature is fixed as a boundary condition. At both mass velocities the liquid film and the vapor core are treated as turbulent; a low-Re form of the SST k-ω model has been used for the modeling of turbulence through both the liquid and vapor phases. Numerical simulations are validated against experimental data. The influence of the surface tension on the shape of the vapor–liquid interface may provide some heat transfer enhancement in a square cross section minichannel, but this depends on the mass flux and it may be not signifi...

A Parametric Study of the Thermal-Hydraulic Performance of a Zigzag Printed Circuit Heat Exchanger

Abstract: The effects of geometric parameters on the performance of a printed circuit heat exchanger have been analyzed using three-dimensional Reynolds-averaged Navier–Stokes equations. The shear stress transport model is used for accurate prediction of the turbulent flows. The numerical solutions are validated in comparison with the available experimental data, and different lengths of the calculation domain have been tested to determine the optimum length of the domain. The effects of two design parameters, namely, the channel angle and the semi-ellipse aspect ratio of the cold channel, on the heat transfer and friction performance in the cold channel have been evaluated. The results indicate that the effectiveness of the heat exchanger is maximized when the cold channel angle is similar to the hot channel angle.

Conjugate Forced Convection Heat Transfer From a Heated Flat Plate of Finite Thickness and Temperature- Dependent Thermal Conductivity

Abstract: In this paper, a conjugate forced convection heat transfer from a good conducting plate with temperature-dependent thermal conductivity is studied. The semi-analytical solution for the nonlinear integro-differential equation occurring in the problem is handled easily and accurately by implementing the differential transform method (DTM). The horizontal plate is heated with uniform heat flux at the lower surface while being cooled at the upper surface under laminar forced convection flow. A numerical approach is also performed via a finite-volume method to examine the validity of the results obtained by DTM. The results of DTM show closer agreement with the results of the numerical method than the results obtained by the perturbation method existing in the literature. It is concluded that for a good conducting plate with a finite thickness the distribution of the conjugate heat flux at the upper surface is significantly affected by the plate thickness. Moreover, we conclude that in the conjugate heat trans...

Experimental Investigation of Al2O3/Water Nanofluid Through Equilateral Triangular Duct With Constant Wall Heat Flux in Laminar Flow

Abstract: This paper experimentally investigates the heat transfer of an equilateral triangular duct by employing an Al2O3/water nanofluid in laminar flow and under constant heat flux conditions to improve the heat transfer performance of this type of duct. The Nusselt numbers were obtained for different nanoparticle concentrations of the nanofluid at various Peclet numbers. The results show that the experimental heat transfer coefficient of Al2O3/water nanofluid is higher than that of distillated water. Also, the experimental heat transfer coefficient of Al2O3/water nanofluid is higher than the theoretical one. The experimental results also indicate that the heat transfer enhancement increases with increases in the nanofluid volume concentration and Peclet number.

Advancements in the Field of Direct Steam Generation in Linear Solar Concentrators—A Review

Abstract: Direct steam generation in parabolic trough or linear Fresnel collectors represents one interesting technological option for concentrating solar electricity production. Today's state of the art characterized by the first commercial plants in operation is a result of more than 20 years of intensive research on this topic. This article provides a review on the key results from research that includes physical effects like heat transfer and pressure drop in horizontal boiler tubes, plant layout considerations, and thermal storage options. An overview on test and demonstration facilities as well as on commercial plants is given, leading to an outlook on the next generation of direct steam generation systems.

Experimental Investigation of Thermal Resistance of a Ferrofluidic Closed-Loop Pulsating Heat Pipe

Abstract: For the present article, a pulsating heat pipe (PHP) is fabricated and tested experimentally by bending a copper tube. The effects of working fluid, heat input, charging ratio, inclination angle, magnets location, and ferrofluid (magnetic nanofluid) volumetric concentration have been investigated on the thermal performance of this PHP. Experimental results show that using ferrofluid as a working fluid improves the thermal performance of the PHP significantly. Moreover, applying a magnetic field on a ferrofluidic PHP reduces its thermal resistance. By changing the inclination angle of the PHP from vertical mode to angles close to the horizontal mode, the present PHP has a constant and acceptable thermal performance. Reduction of the magnetic flux density at the evaporator decreases the PHP's thermal performance as well. Increasing the ferrofluid volumetric concentration in the experimental range enhances the thermal performance of the PHP.

Modeling of Thermal Energy Storage Shell-and-Tube Heat Exchanger

Abstract: This paper reports on the development of a computationally efficient numerical simulation model for a shell-and-tube thermal energy storage system, where the heat transfer occurs between a fixed mass of phase-change material (PCM) in contact with a tube through which flows a high-temperature fluid. Simulations of the conjugate heat transfer and melting/solidification of the PCM for a range of heat exchanger designs, including single-tube control/baseline, single tube with longitudinal and circular fins, and multitube configurations, were undertaken and the numerical results were validated using experimental data. The underlying simplifications in the model were tested and verified against two- and three-dimensional simulations. During charging, before complete melting of the PCM, the PCM volume average temperature is predicted to be within 5°C of the experimentally determined value for each heat exchanger design. During discharging there is a good trend-wise agreement between predicted and measured temper...

Simultaneous Measurement of Three-Dimensional Temperature Distributions and Radiative Properties Based on Radiation Image Processing Technology in a Gas-Fired Pilot Tubular Furnace

Abstract: An on-line three-dimensional temperature measurement experiment was carried out in a gas-fired pilot tubular furnace. Four flame image detectors were utilized to obtain two (red and green) monochromatic radiation intensity distributions, which can be calculated by the DRESOR method based on the radiation image processing technology. Then a revised Tikhonov regularization method was developed to reconstruct three-dimensional temperature distributions from the green monochromatic radiative intensity. Meanwhile, a Newton method combined with a least-squares method was used to simultaneously reconstruct radiative properties from the red one. The two calculation procedures were performed alternately, forming an iterative algorithm to a simultaneous reconstruction of temperature and radiative properties. The reconstructed temperatures agreed well with those measured by thermocouples for different cases with different calorific values and components of gas. The largest relative error was less than 3%, which vali...

Engineering Thermal and Mechanical Properties of Multilayer Aligned Fiber-Reinforced Aerogel Composites

Abstract: This paper demonstrates a controllable way to fabricate multilayer aligned fiber-reinforced aerogel composites with low thermal conductivity (e.g., 0.022–0.028 W m−1 K−1) and greatly improved bending and compressive strengths compared to pure aerogels. Four-layer aligned fibers reinforced aerogel composites with six laminated structures were successfully synthesized by impregnating four layers of aligned glass fibers in silica aerogels via sol–gel technique and drying at ambient pressure. The fiber alignments and the laminated structures greatly affected the compressive and bending strengths of the aerogel composites. For instance, as the orthogonal number of the fiber layers was increased, the bending strength and stability of the aerogel composites were improved while the compressive strength of the aerogel composites was degraded. A heat transfer model based on the unit cell of surface contact hollow cubic structure and the parallel law of equivalent thermal resistance concerning the as-prepared aeroge...

A Framework for the Analysis of Thermal Losses in Reciprocating Compressors and Expanders

Abstract: This article presents a framework that describes formally the underlying unsteady and conjugate heat transfer processes that are undergone in thermodynamic systems, along with results from its application to the characterization of thermodynamic losses due to irreversible heat transfer during reciprocating compression and expansion processes in a gas spring. Specifically, a heat transfer model is proposed that solves the one-dimensional unsteady heat conduction equation in the solid simultaneously with the first law in the gas phase, with an imposed heat transfer coefficient taken from suitable experiments in gas springs. Even at low volumetric compression ratios (of 2.5), notable effects of unsteady heat transfer to the solid walls are revealed, with thermally induced thermodynamic cycle (work) losses of up to 14% (relative to the work input/output in equivalent adiabatic and reversible compression/expansion processes) at intermediate Peclet numbers (i.e., normalized frequencies) when unfavorable solid a...

Solar Powered Cascading Cogeneration Cycle with ORC and Adsorption Technology for Electricity and Refrigeration

Abstract: As a renewable source, solar energy has received more and more attention in recent years. Solar energy can readily provide heat efficiently within the temperature range of 70–100°C. For the utilization of this energy source, a cascading cycle was designed and was discussed. An organic Rankine cycle (ORC) and an adsorption refrigeration cycle were combined to provide the first- and second-stage energy conversion cycle, respectively. In the analysis, R600 was used as the working fluid for the ORC and a silica gel–water working pair was analyzed for the adsorption refrigeration cycle. The energy efficiency for electrical generation and refrigeration, as well as the exergy efficiency of the cascading cycle, was assessed. For an environmental temperature of 30°C and a refrigeration temperature of 12°C, the results showed that typically 1 kW of electricity and 6.3 kW of refrigeration could be generated from approximately 15 kW heating power. The electricity generation efficiency was between 0.1 and 0.15, while ...

Moving Bed Heat Exchangers for Use With Heat Storage in Concentrating Solar Plants: A Multiphase Model

Abstract: Heat storage based on particulate materials is a promising option to provide a demand-oriented electricity production with utility-scale solar power plants. For energy storage discharge, a moving bed heat exchanger is considered and its design is investigated. As a basis for a flexible design tool, a multiphase model based on the Eulerian continuum approach was set up to describe the bulk flow and the thermal performance. The model was applied to an example heat exchanger layout, and the simulation results were compared with an empirical model, confirming the validity of the approach. Initial parameter variations identify the key parameters and their effect on the thermal performance.

Natural Convection Heat Transfer of Copper–Water Nanofluid in a Square Cavity With Time-Periodic Boundary Temperature

Abstract: This paper presents a numerical study of natural convective heat transfer of copper–water nanofluid in a square enclosure where the temperature of the left vertical sidewall is sinusoidally oscillated with a constant average temperature, the right sidewall is cooled at a relatively low temperature, and the other walls are kept adiabatic. The influence of pertinent parameters such as Rayleigh number, solid volume fraction of copper nanoparticles, and dimensionless time period on the heat transfer characteristics is studied. The results show that the heat transfer rate increases using copper nanoparticles.

3D ALE Finite-Element Method for Two-Phase Flows With Phase Change

Abstract: We seek to study numerically two-phase flow phenomena with phase change through the finite-element method (FEM) and the arbitrary Lagrangian–Eulerian (ALE) framework. This method is based on the so-called “one-fluid” formulation; thus, only one set of equations is used to describe the flow field at the vapor and liquid phases. The equations are discretized on an unstructured tetrahedron mesh and the interface between the phases is defined by a triangular surface, which is a subset of the three-dimensional mesh. The Navier–Stokes equation is used to model the fluid flow with the inclusion of a source term to compute the interfacial forces that arise from two-phase flows. The continuity and energy equations are slightly modified to take into account the heat and mass transport between the different phases. Such a methodology can be employed to study accurately many problems, such as oil extraction and refinement in the petroleum area, design of refrigeration systems, modeling of biological systems, and effi...

CFD Simulation of Heat Transfer and Pressure Drop in Compact Brazed Plate Heat Exchangers

Abstract: In this paper, the thermal and hydraulic characteristics of corrugated fluid channels of compact brazed plate heat exchangers (BPHE) are investigated by computational fluid dynamics (CFD) simulations using the commercial CFD software ANSYS CFX 14.0. The influence of geometry parameters of the corrugated pattern such as chevron angle and corrugation pitch on the BPHE performance is investigated on small fluid section geometries. The influence of various types of wall heat transfer boundary conditions on the simulation results is also studied. An entire fluid channel is simulated using various turbulence models in the Reynolds number range of 300 to 3000. The CFD predictions are also validated using data obtained from laboratory experiments. The simulations of the entire fluid channel underpredict heat transfer and pressure drop by 20–30% and 10–35%, respectively. The results from the small fluid sections suggest that the CFD simulations can be used as a reasonably effective tool in determining the relative...

Performance Analysis of Flat-Plate Solar Collector Having Silver Nanofluid as a Working Fluid

Abstract: A study on water solar collector performance having silver nanofluid as working fluid was carried out. In this study, 20-nm silver particles mixed with water at the concentrations of 1,000 and 10,000 ppm were undertaken in 3 small identical closed-loop flat-plate solar collectors, each with an area of 0.15 m × 1.0 m. The mass flux of the working fluid varied between 0.8 and 1.2 L/min-m2 and the inlet temperatures were controlled in the range of 35–65°C. The tests were performed outdoor under a steady-state condition. The experimental results showed that at the same Reynolds number, the convective heat transfer coefficient of the nanofluid inside the solar absorber tube at 1,000 ppm was slightly higher than that of water, and at 10,000 ppm, the heat transfer coefficient was about 2 times that of water. This meant that the overall heat loss coefficient of the solar collector with nanofluid could be reduced and more solar heat gain could be obtained, especially with a high inlet temperature of the working fl...

Numerical Simulation of Transpiration Cooling for Sintered Metal Porous Strut of the Scramjet Combustion Chamber

Abstract: The strut structure in a scramjet combustion chamber is used to inject fuel into the main stream. The environment surrounding the strut in the scramjet chamber is supersonic flow at very high temperatures. Thus, the leading edge of the strut is easily ablated due to aerodynamic heating. This study analyzes the effect of a transpiration cooling scheme using a sintered metal porous media surface to protect the strut from ablation. Numerical simulations are used to study the transpiration cooling for different strut structures and coolant conditions. The influences of these parameters on the transpiration cooling of the strut are analyzed for a main stream Mach number of 2.5 and a total temperature of 1700 K. The surface temperature can be reduced to a safe temperature with a coolant mass flow rate through the porous media of 27.5 kg/ m2-s. The coolant flow near the leading edge is most important, with less flow needed downstream.

Heat Transfer and Pressure Drop Characteristics of Smooth Horizontal Tubes in the Transitional Flow Regime

Abstract: The operating conditions of many heat exchangers are in, or close to, the transitional flow regime. However, in this regime, not a lot of design information is available and some design books even recommend to not design heat exchangers to operate in the transitional flow regime. Furthermore, it is known that the type of inlet of heat exchangers influences the transition characteristics. It was therefore the purpose of this study to measure heat transfer and pressure drop characteristics in smooth horizontal tubes using different types of inlets. The types of inlets were hydrodynamically fully developed, square-edged, re-entrant, and bellmouth. Experiments were conducted on a 14.48-mm inner diameter horizontal tube in which the water was cooled. Reynolds numbers ranged between 1000 and 20,000 and Grashof numbers were on the order of 105. It was found that for adiabatic flow the square-edged inlet delayed transition to Reynolds numbers of around 2600, while the bellmouth inlet delayed it to about 7000. How...

Thermal Performance of an Air-Cooled Data Center With Raised-Floor and Non-Raised-Floor Configurations

Abstract: In this paper, the thermal performances of an air-cooled data center with raised-floor and non-raised-floor configurations are compared with respect to the room and ceiling return strategies. The thermal performance of the data center is evaluated in terms of supply heat index, rack cooling index, total irreversible loss, and the number of racks with at least one server exceeding the maximum recommended and allowable inlet air temperature according to American Society of Heating, Refrigeration, and Air-Conditioning Engineers (ASHRAE) thermal guidelines. The numerical simulations are conducted providing an insight into the flow and temperature distributions, and thus giving a better understanding of the cooling issues. It is found that using a ceiling return strategy for the return of hot exhaust air to the computer room air conditioning units gives a better thermal performance of the data center, for both raised- and non-raised-floor strategy, as compared to the room return. The findings are then extended...

Flow Boiling Heat Transfer in A Minichannel with Enhanced Heating Surface

Abstract: This paper presents the results of flow boiling in a 1.0-mm-deep minichannel with asymmetrical heating. The heating element for the working fluid (FC-72) is a single-sided enhanced alloy foil made from Haynes-230. Two types of enhanced heating surfaces, prepared by laser texturing and with microrecesses varied in terms of size, were used for investigations. The experimental research focused on the transition from single-phase forced convection to nucleate boiling, that is, the zone of boiling incipience and further development of boiling. Flow structure was observed through a glass pane. Owing to the liquid crystal layer placed on the opposite side of the enhanced foil surface, it was possible to observe the onset of flow boiling (as a “boiling front”) and to measure temperature distribution on the heating wall through another glass pane. The objective of the study is to determine void fractions for increasing heat fluxes supplied to the heating surface. The flow structure photos were processed in Corel g...

Energy-Efficient Building Envelopes with Phase-Change Materials: New Understanding and Related Research

Abstract: Thermal performance of building envelopes is important in improving building energy efficiency. This paper includes three parts: (1) It introduces our new understanding of developing optimized or ideal building envelopes based upon inverse problems and their solutions, which is helpful for energy efficient building envelope structure design and material development. (2) It reviews our recent research on developing energy-efficient building envelopes with phase-change materials, including the optimized thermal mass characteristics of building wall, novel phase-change material development, measurement method for thermal physical properties of phase-change material, and application of some prototype energy efficient building envelopes. (3) It puts forward some limitations of the available works and related research topics for future study.

Mathematical Modeling of Cross-Flow Tube Heat Exchangers With a Complex Flow Arrangement

Abstract: The general principles of mathematical modeling of heat transfer in cross-flow tube heat exchangers with complex flow arrangements that allow the simulation of multipass heat exchangers with many tube rows are presented. The finite-volume method is used to solve the system of differential equations for temperature of the both fluids and the tube wall with appropriate boundary conditions. A numerical model of a multipass steam superheater with 12 passes is presented. The convection and radiation heat transfer on the flue gas side are accounted for. In addition, the deposit layer is assumed to cover the outer surface of the tubes. Comparing the computed and measured steam temperature increase over the entire superheater allows for determining the thermal resistance of the deposits layer on the outer surface of the superheater. The developed modeling technique can especially be used for modeling tube heat exchangers when detailed information on the tube wall temperature distribution is needed.

Heat Transfer and Friction Factor With Water/Propylene Glycol-Based CuO Nanofluid in Circular Tube with Helical Inserts Under Transition Flow Regime

Abstract: The article deals with the transition flow convective heat transfer and friction factor characteristics of water/propylene glycol-based CuO nanofluids flowing in a horizontal circular tube fitted with and without helical inserts. CuO nanoparticles of average particle diameter less than 50 nm were suspended in the base fluid and nanofluids of three different concentrations were prepared. Experiments were also conducted by inserting helical inserts having twist ratio in the range of 0 to 9 and Reynolds number ranging from 2500 to 10,000. The Nusselt number obtained with 0.5% concentration of CuO nanofluids is about 28% higher in a plain tube and is increased further up to 5.4 times over the base fluid when the helical insert with twist ratio 3 is used. In the plane tube the friction factor is increased by 10% and further enhanced to 140% with the usage of helical insert compared to the friction factor obtained with the base fluid alone. The friction factor penalty is very much less compared to the benefit o...

Optimized Integration of Renewable Energy Technologies into Jordan’s Power Plant Portfolio

Abstract: Jordan has experienced a significant increase of peak load and annual electricity demand within the last years due to economic development and population growth. The experienced growth rates are expected to continue during the next decades, making large investments in new power plant capacity necessary. Additionally, when gas supply from Egypt was interrupted several times and crude oil world market prices increased simultaneously, recent years have shown painfully that a power supply exclusively based on fossil fuel imports is subject to a very high risk and can have a strong negative impact on the national budget. Electricity-sector authorities are therefore looking for suitable solutions to keep up with the increasing electricity demand, to make Jordan more independent from fossil fuel imports, and to provide electricity at reasonable prices in the future. This paper presents a methodology for the optimized integration of renewable energy (RE) technologies into Jordan's existing power plant portfolio. ...

Lattice Boltzmann Method Applied to Radiative Transport Analysis in a Planar Participating Medium

Abstract: This article deals with the extension of the usage of the lattice Boltzmann method (LBM) to the analysis of radiative heat transfer with and without conduction in a one-dimensional (1-D) planar participating medium. A novel lattice needed for the calculation of the volumetric radiation spanned over the 4π spherical space has been introduced. The LBM formulation is tested for three benchmark problems, namely, radiative equilibrium, nonradiative equilibrium, and a combined mode conduction–radiation problem in a planar geometry. In the combined mode problem, with radiative information known from the proposed lattice structure, the energy equation is also formulated and solved using the LBM. The D1Q2 lattice is used in the energy equation. For validation, in problems 1 and 2, the LBM results are compared with the finite-volume method (FVM), while in problem 3, the LBM-LBM results are compared with the LBM-FVM in which FVM is used for the computation of radiative information. Comparisons are made for the effec...

Experimental and Numerical Study of Mixed and Natural Convection in an Enclosure With a Discrete Heat Source and Ventilation Ports

Abstract: An experimental and numerical study has been carried out in order to investigate mixed and natural convection heat transfer in a two-dimensional enclosure. A discrete isothermal heat source is located at one of the vertical walls. Also, two ventilation ports are at the bottom and on top of the opposite wall. A forced flow condition was imposed by providing an inlet of air at the bottom port. A Mach–Zehnder interferometer was used to visualize the temperature field within the enclosure and to determine the local and average heat transfer characteristics of the heat source. Five heater positions on the vertical wall and different Rayleigh numbers (4.5 × 105 to 1.15 × 106) and Reynolds numbers (120 to 1600) were considered in the experiments. A finite volume code has been developed based on the SIMPLE algorithm and hybrid discretization scheme for the numerical study. It is observed that the interaction of natural convection with the forced flow leads to various flow fields depending on the Richardson number...

Experimental Studies for Convective Drying of Potato

Abstract: An experimental facility was built at the Indian Institute of Technology Delhi in order to examine the characteristics of convective drying of a moist object. The test facility consists of an inlet section, a divergent and convergent section, a settling chamber, a test section, and an outlet section. Initial moisture content and time-dependent moisture content of a rectangular shaped moist object (4 cm × 2 cm × 2 cm) are measured by this test facility. The potato slice was selected as a sample moist object. Moisture content was measured at different air temperatures of 40, 50, 60, and 70°C with an air velocity of 2 m/sec. The density of potato slice was determined for various drying temperatures. The volume shrinkage during drying decreased almost linearly with moisture content. The percentage air pores and porosity increased gradually with decreasing moisture content and increasing drying air temperature. Volumes of water, air, and solid content of potato were determined at different drying air temperatu...

Evaluation of the Heat Transfer Correlations for Supercritical Pressure Water in Vertical Tubes

Abstract: A large number of studies have been carried out on the flow and heat transfer of supercritical pressure fluids in the past decades. However, there are still some uncertainties and deficiencies in the accurate prediction for supercritical fluid heat transfer coefficient due to the large and fast variations of fluids properties in the so-called pseudo-critical region. In this paper, 15 correlations were selected from the literature and were compared with each other to verify their capability in predicting heat transfer coefficient of supercritical pressure water in vertical tubes. Based on the comparison between the calculation results of the existing heat transfer correlations and the experimental data obtained from the open literature, it was found that the Swenson et al. correlation and the Hu correlation can reasonably predict the heat transfer coefficient of supercritical water in the pseudo-critical region. After evaluating these correlations, the authors conducted polynomial fitting for the collected...

Pool boiling on modified surfaces using R-123

Abstract: Saturated pool boiling of R-123 was investigated for five horizontal copper surfaces modified by different treatments, namely, an emery-polished surface, a fine sandblasted surface, a rough sandblasted surface, an electron beam-enhanced surface, and a sintered surface. Each 40-mm-diameter heating surface formed the upper face of an oxygen-free copper block, electrically heated by embedded cartridge heaters. The experiments were performed from the natural convection regime through nucleate boiling up to the critical heat flux, with both increasing and decreasing heat flux, at 1.01 bar, and additionally at 2 bar and 4 bar for the emery-polished surface. Significant enhancement of heat transfer with increasing surface modification was demonstrated, particularly for the electron beam-enhanced and sintered surfaces. The emery-polished and sandblasted surface results are compared with nucleate boiling correlations and other published data.