Maximum Heat Transfer Performance Research Articles

Cooling performance study of a novel heat exchanger in an absorption system

The paper is focusing on a shell and plate heat exchanger of a novel absorption refrigeration system. The system is composed of two vacuum vessels connected together with a steam channel and one heat exchanger is located in each vessel. The first heat exchanger is called reactor where working fluid and salt exist and the second heat exchanger or evaporator/condenser (C/E) is where only water exists. The propylene glycol-based (PG) heat transfer fluid is used on the shell side of both heat exchangers as the media to exchange the heat between boilers and reactor in one vessel and between cold environment and condenser/evaporator in another vessel. An experimental test rig was built to investigate the performance of the evaporator/condenser heat exchanger. Then, a three-dimensional (3D) Computational Fluid Dynamics (CFD) model was developed. The experimental result was then used to validate the numerical model developed by using Ansys/Fluent software. A parametric study has been intended to find a more appropriate design for the heat exchanger in order to increase heat transfer performance. Results of the parametric study demonstrated that the cooling performance is doubled by increasing the diameter of the plate from 0.14 m to 0.2 m. In addition, to obtain the maximum heat transfer performance, Reynolds number and distance between plates should be 9 and 0.5 m, respectively. Two correlations have been developed for the outlet temperature and cooling power of the heat exchanger which are functions of heat transfer coefficient. The results of this study can be of vital importance for improving the cooling power of the system, remarkably.

Ultrasonic Characterization of Silver Nano Fluid

Nano fluids are stable suspensions of nano particles in a liquid. Metal oxide nanoparticles offer great merits over controlling rheological, thermal, chemical and physical properties of solutions. The effectiveness of a nanoparticle to modify the properties of a fluid depends on its diffusive properties with respect to the fluid. An important application of Nano fluids containing nano particles is as a coolant, since the addition of only a few volume percent of nano particles to a liquid coolant and significantly improves its thermal conductivity. A heat exchange device using nanofluid needs to operate at best nanoparticle loading to get the maximum heat transfer performance. Due to its antimicrobial properties, the future therapeutic directions may include nano-silver as an anti-inflammatory and as anti-platelet agents and antiviral drugs. Owing to the applications of nano silver particles, attempts has been made in the present study to synthesis silver nano particles and characterized with the help of ultrasonic technique. Different acoustic properties of AgNO3 nanofluids are studied with the help of Ultrasonic technique. Silver nanofluids are synthesized by chemical reduction method. The chemical reduction method involves the reduction of AgNO3 in aqueous solution by a reducing agent in the presence of a suitable stabilizer, which is necessary in protecting the growth of silver particles from aggregation. Ultrasonic velocity and density values are measured for different concentrations of AgNO3 nanofluids. The acoustic parameters like adiabatic compressibility, intermolecular free length, and acoustic impedance are calculated from the experimental data. The variation of ultrasonic velocity and other parameters with particle concentration are discussed on the basis of interaction of ultrasonic wave with the nano sized silver particles.

Heat transfer on topographically structured surfaces for power law fluids

The three-dimensional power law fluid flow through rough microchannels has been studied numerically to determine the effects of the topographic structures on the thermal and hydrodynamic characteristics of the system. Rectangular, triangular and sinusoidal element shapes have been considered in order to investigate the effects of roughness height, width, pitch and channel separation on the pressure drop and heat transfer. Uniform wall heat flux boundary condition has been applied for all the peripheral walls.The results indicate that the global heat transfer performance can be improved or reduced by the roughness elements at the expense of pressure head when compared with the smooth channels. The maximum heat transfer performance has been obtained for triangular roughness shapes with the relative roughness height of 0.333. In most cases heat performance is smaller than unity. It is also revealed that for the sinusoidal and triangular cases by decreasing the relative roughness width, heat transfer performance is improved. Furthermore, the effects of the roughness features and the channel separation are intensified in power-law fluids. For dilatant fluids with a power-law index equal to n = 1.25, channel separation ratio reduction causes a steep rise in normalized friction factor up to 140.8 in the rectangular case. By increasing the relative roughness pitch of the cases with rectangular elements and the power-law index of n = 0.75, a reasonable heat transfer performance enhancement has been observed.

Numerical study of heat transfer enhancement by rectangular winglet vortex generator pair in a channel

A numerical simulation was performed to investigate the effects of longitudinal vortices on the heat transfer enhancement of a laminar flow in a rectangle duct mounted with rectangular winglet pair on the bottom wall. The studied Reynolds number which was defined using the hydraulic diameter twice the channel height ranges from 500 to 7000. The comparisons of the fluid flow and heat transfer characteristics for the cases with and without rectangular winglet pair were carried out. The effects of the height and attack angle of vortex generator pair on the heat transfer performance were investigated. The results show that mounting rectangular winglet pair on the bottom wall of the channel can significantly enhance heat transfer. The distributions of secondary flow on the cross sections are consistent with the distributions of Nu and J for different attack angles. The maximum heat transfer performance is obtained when the attack angle is 29° due to the maximum value of secondary flow generated by rectangular winglet pair.

A synthetic layout optimization of discrete heat sources flush mounted on a laminar flow cooled flat plate based on the constructal law

A synthetic optimization is presented for the Pareto layouts of discrete heat sources (with uniform heat flux) flush mounted on a flat plate over which laminar flow serves for cooling purpose. The peak temperatures and the flow drag loss are minimizing simultaneously provided that the total heat dissipation rate and the plate length are held constant. The impact of the manufacturing limit, i.e. the minimum length of the heated or the adiabatic patch, on the optimum layout is discussed. The results in general comply with analytical deduction based on the constructal theory. However in a finite length scenario, geometric constraints on the adiabatic spacing differ from that fits the situation in which maximum heat transfer performance alone is to be achieved.

Gas-Cooled Divertor Concept with High Thermal Conductivity Porous Media

In this study, in order to enhance heat transfer performance of helium gas flow for divertor cooling, high thermal conductivity porous media that are copper-particles-sintered ones are introduced as the referential porous media. In order to predict the heat transfer performance of He gas impinging jet flow with the porous medium, nitrogen gas is used as the simulant of helium gas in the pressure range of 0.1 MPa to 0.8 MPa. With the porous medium, the particle introduced is highly size-adjusted one of 1000 μm in diameter and the porosity is almost 30 %. The maximum heat transfer performance is evaluated by numerically simulating temperature field in a heat transfer block based on the measured temperature data. The experiments prove that the heat transfer coefficient of N2 gas impinging jet flow with the porous medium is much higher than that of common impinging jet flow without the porous medium from the view point of not only flow velocity but also pumping power.

Particle concentration levels of various nanofluids in plate heat exchanger for best performance

A heat exchange device using nanofluid needs to operate at best nanoparticle loading to get the maximum heat transfer performance. In this paper, an attempt has been made to optimize different nanofluid particle volume fractions based on a maximum heat transfer rate, convective heat transfer coefficient, overall heat transfer coefficient, effectiveness and performance index. The novelty of the present study is the optimization of particle volume fraction of various nanofluids based on experimentation in the commercial plate heat exchanger for wide range of nanoparticle volume fraction (0–3%). Effects of other operating conditions on the optimization have been discussed as well. Results show that for maximum enhancement of heat transfer characteristics, different nanofluids work at different optimum volume concentrations. For CeO2/water, Al2O3/water, TiO2/water and SiO2/water nanofluids, the optimum volume concentrations are 0.75%, 1%, 0.75% and 1.25%, respectively, at the flow rate of 3lpm. The corresponding maximum heat transfer enhancements are about 35.9%, 26.3%, 24.1%, and 13.9%, respectively. Present study indicates that the optimum concentration for maximum performance index is lower than that for maximum heat transfer rate. Moreover the effect of nanomaterials is more predominant than that of heat transfer fluid temperature or volume flow rate.

Fluid flow and heat transfer investigations on enhanced microchannel heat sink using oblique fins with parametric study

Abstract Enhanced microchannel heat sink with sectional oblique fin is used to modulate the flow in contrast to continuous straight fin. The re-initialization of thermal boundary layer at the leading edge of each oblique due to breakage of continuous fin into oblique sections and the secondary flow due to these oblique cuts resulted in better heat transfer and a comparable pressure drop. Extensive experimental investigations are carried out with silicon test vehicle with hydraulic diameter of 100 μm and 200 μm and de-ionized water as flowing fluid. A parametric study involving the oblique angle, fin pitch is also carried out. Appreciable heat transfer augmentation is also achieved with maximum heat transfer performance enhancement at 47% when Re = 680. Comparable pressure drop to conventional microchannel is maintained up to Re = 500. Parametric study suggests that smaller oblique angle and smaller fin pitch are beneficial for heat transfer enhancement. The performance of the microchannel with 100 μm channel width and 27° oblique angle is found to be optimum.

Optimization in plate-fin safety structure of heat exchanger using genetic and Monte Carlo algorithm

A safety structure of plate-fin heat exchanger is designed for special applications to prevent fluid leakage from adjacent channel walls. A fractional volume of a cavity layer between two channels is filled with high thermal conductive column-shape metal. Genetic algorithm is used for optimization of column distributions to achieve the maximum heat transfer performance, and its output is better than the simple direct optimization. To optimize with uncertain fluid condition, a direct genetic algorithm method, two improved genetic algorithm methods and a specific type of Monte Carlo algorithm method are applied in searching suitable solution. The optimized structure can provide a new feasible and safety plate-fin heat exchanger, and its results obtained by using genetic algorithm and Monte Carlo algorithm can provide some guidelines for optimal designs of heat exchangers.

Heat transfer and capillary performance of dual-height superhydrophilic micropost wicks

We propose dual-height superhydrophilic (DHS) micropost evaporator wicks with improved heat transfer performance for thermal management applications such as micro heat pipes. The heat transfer coefficient and capillary performance are characterized with a numerical model that accounts for the finite curvatures of liquid menisci by varying solid fraction of micropost arrays from 0.18 to 0.54. The DHS wicks vertically stretch a thin (<2μm) evaporative film region with low thermal resistance, which enhances the heat transfer coefficient up to ∼300% compared with the previously reported single-height superhydrophilic (SHS) micropost wicks with the same solid fraction. The stretch of thin film region does not significantly affect the capillary pressure and permeability determined from microscopic menisci. The optimum solid fraction maximizing the heat transfer coefficient occurs since the maximum height ratio between tall and short posts satisfying the pinning criteria decreases with increasing solid fraction, while the total perimeter of microposts increases. The vertical stretch of thin film also lowers the sensitivity of performance to liquid fill charge over 60%. This work suggests that DHS micropost wicks can provide significantly higher heat transfer coefficient in more robust way compared with previous SHS wicks without scarifying the maximum heat transfer performance.

데시칸트 제습기용 열교환기 설계에 관한 수치해석적 연구

A numerical analysis is performed to evaluate mass flow balance in the heat exchanger for the dehumidifier. To improve the mass flow balance for maximum heat transfer performance, inlet, outlet and baffle are changed. Mass flow balance is evaluated by non-uniformity of flow which is the same concept with the standard deviation. Usually, there will occur many paths between the inlet and the outlet, however, it will follow shortest and low resistance ways. The uniform distribution of flow is numerically analyzed for several types of heat exchangers. Making the shortest way between the inlet and the outlet is most important factor. Two types of heat exchangers are installed in the dehumidifier and 4 cases of Type A heat exchangers and 3 cases of Type B heat exchangers are evaluated and optimized. The result of this research is applied to design heat exchanger for commercial dehumidifiers.

Hydraulic and thermal performances of a novel configuration of high temperature ceramic plate-fin heat exchanger

A novel fin configuration for high temperature ceramic plate-fin heat exchanger (PFHE) was developed using the three-dimensional computational fluid dynamics (CFD) FLUENT code. Numerical analysis was carried out for different types of fins and their results were compared with the selected design. The working fluids used in the model were sulfur trioxide, sulfur dioxide, oxygen and water vapor. Fluid flow, heat transfer, pressure drop and properties like Nusselt number, friction factor and j-factor were studied for various fin configurations. The rip saw fin design (case 9) with thickness of 0.05mm gives the maximum heat transfer performance with less pressure drop and friction factor. The numerical result was compared with the analytical result for rectangular fins and they were found to be in reasonable agreement. In addition to it, the results from the selected ripsaw design were compared with the result from the model with no fins (case 1). It was found that thermal enhancement factor of 2.3211 and average Nusselt number of 4.215 was obtained for the selected design. The results of the rip saw fin design were found in good agreement with the analytical results of a rectangular fin. Further effects of Reynolds number on pressure drop and Nusselt number were studied.

Investigating the heat transfer performance and thermophysical properties of nanofluids in a circular micro-channel

In this paper, the thermal performance of a circular shaped copper microchannel heat sink using three types of nanofluids is discussed analytically. Al2O3–Water, TiO2–water and CuO–water nanofluids were used in this analysis and the comparative thermal performance of these three nanofluids is also discussed. The hydraulic diameter of the circular channel is 400μm and the total block dimension is 10mm×10mm×4mm. A steady, laminar and incompressible flow with constant heat flux is assumed in the circular channel. The analyses are done at various volume fractions ranging from 0.5vol.% to 4vol.% and at a constant inlet velocity of 1.5m/s. The results showed that the thermal performance can be increased significantly by using CuO–water nanofluid as a coolant for cooling of electronic heat sink when Al2O3–water and TiO2–water nanofluids showed less improvement. Compared to pure water, the highest improvement (13.15%) in the heat flux occurred for 4vol.% CuO–water nanofluid when Al2O3–water and TiO2–water nanofluids showed 6.80% and 6.20% improvements respectively. This improvement in heat flux is calculated without considering the additional required pumping power due to the increased viscosity of nanofluids. Therefore, CuO–water nanofluid can be recommended to obtain maximum heat transfer performance in a circular microchannel heat sink.

Combined Heat Transfer of Radiation and Convection on Wall Sintered with Open Celled Metallic Foams

Experimental investigation of the combined radiation and natural convection heat transfer on wall surface sintered with open celled metal foams at different inclination angles is presented.The effects of wall inclination angle,Rayleigh number Ra* and thickness on heat transfer performance are studied.The heat transfer results for vertically placed smooth cooper plate are in good agreement with the available data in the literature to show the feasibility of the test rig.With increasing of the inclination angle under the same heat flux,the pure natural convection heat transfer coefficient and total heat transfer coefficient increase firstly to a turning point and then decrease.The maximum heat transfer performance occurs at an inclination angle between 60° and 80°.The radiation heat transfer accounts for 33%～44% of the total heat transfer.When placed vertically,the ratio of radiation heat transfer of metallic foam sintered surface in the total heat transfer is less than that of the smooth surface,and is improved with increasing of metal foam thickness.Compared to the smooth surface,the mean Nusselt numbers of combined heat transfer and pure natural convection heat transfer of vertically placed sintered surface increase by 1.52～1.98 and 1.16～1.66 times respectively.The temperature distribution on foam surface photographed by infrared camera are analyzed with image processing technique to validate the thermal radiation heat transfer calculation model based on average foam surface temperature.

Thermo-Hydraulic Behavior of Microchannel Heat Exchanger System

This article presents an experimental study of thermo-hydrodynamic phenomena in a microchannel heat exchanger system. The aim of this investigation is to develop correlations between flow/thermal characteristics in the manifolds and the heat transfer performance of the microchannel. A rectangular microchannel fabricated by a laser-machining technique with channel width and hydraulic diameter of 87 μm and 0.17 mm, respectively, and a trapezoidal-shaped manifold are used in this study. The heat sink is subjected to iso-flux heating condition with liquid convective cooling through the channels. The temporal and spatial evolutions of temperature as well as total pressure drop across the system are monitored using appropriate sensors. Data obtained from this study were used to establish relationships between parameters such as longitudinal wall conduction factor, residence and switching time, and thermal spreading resistance with Reynolds number. Result shows that there exist an optimum Reynolds number and conditions for the microchannel heat exchanger system to result in maximum heat transfer performance. The condition in which the inlet manifold temperature surpasses the exit fluid temperature results in lower junction temperature. It further shows that for a high Reynolds number, the longitudinal wall conduction parameter is greater than unity and that the fluid has sufficient dwelling time to absorb heat from the wall of the manifold, leading to high thermal performance.

Effect of nanoparticles in nanofluid on thermal performance in a miniature thermosyphon

An experiment was performed to investigate the effect of nanoparticles in the nanofluid on the thermal performance in a miniature thermosyphon. The nanofluids consisted of de-ionized water and CuO nanoparticles having an average size of 30nm. The experimental results show that the water-CuO nanofluids can greatly enhance the boiling heat transfer performance of the evaporator in thermosyphon compared with that using water at subatmospheric pressure conditions. A much lower and more uniform wall temperature of the thermosyphon can be obtained by substituting the nanofluids for water. Boiling heat transfer coefficients and the critical heat flux (CHF) of the nanofluids in the evaporator of the thermosyphon have significant increase compared with those of de-ionized water. There was an optimal mass concentration which was estimated to be 1.0wt% to achieve the maximum heat transfer performance. Operating pressure has very remarkable influences on both the heat transfer coefficients and the CHF of nanofluids, which greatly increase with the decrease of the test pressure. The heat transfer coefficient and the CHF can increase, respectively, about 160% and 120% at the pressure of 7.45kPa compared with those of water. The experimental study confirmed that the heat transfer performance of the miniature thermosyphon can evidently be strengthened by using water-CuO nanofluids.

Multi-objective optimization of the sandwich panels with prismatic cores using genetic algorithms

In the present study, a new type of sandwich panels with prismatic cores, which are capable of load bearing as well as cooling, is optimized to have minimum weight and maximum heat transfer performance. In order to simultaneously minimize the total weight and maximize the heat transfer performance, a multi-objective optimization approach has been developed using genetic algorithms. A set of compromised solutions, known as the tradeoff surface, is obtained. The tradeoff information between the two objectives is exploited in terms of multi-functionality of the sandwich panels, and the relation between the two objectives is quantified in the present study. The detailed configurations and dimensions of the sandwich panels at the optima are provided. Some basic characteristics of the sandwich panels with prismatic cores have been observed in terms of their multi-functionality.

USPTO experience in training examiners on the Automated Patent System (APS)

A heat exchange device using nanofluid needs to operate at best nanoparticle loading to get the maximum heat transfer performance. In this paper, an attempt has been made to optimize different nanofluid particle volume fractions based on a maximum heat transfer rate, convective heat transfer coefficient, overall heat transfer coefficient, effectiveness and performance index. The novelty of the present study is the optimization of particle volume fraction of various nanofluids based on experimentation in the commercial plate heat exchanger for wide range of nanoparticle volume fraction (0–3%). Effects of other operating conditions on the optimization have been discussed as well. Results show that for maximum enhancement of heat transfer characteristics, different nanofluids work at different optimum volume concentrations. For CeO2/water, Al2O3/water, TiO2/water and SiO2/water nanofluids, the optimum volume concentrations are 0.75%, 1%, 0.75% and 1.25%, respectively, at the flow rate of 3 lpm. The corresponding maximum heat transfer enhancements are about 35.9%, 26.3%, 24.1%, and 13.9%, respectively. Present study indicates that the optimum concentration for maximum performance index is lower than that for maximum heat transfer rate. Moreover the effect of nanomaterials is more predominant than that of heat transfer fluid temperature or volume flow rate.