Heat Transfer Results Research Articles

Vibrational effect on internal heated porous medium in the presence of chaos

The present article investigates a chaotic convection under modulation in the presence of internal heat generation. The viscoelastic fluid saturating porous medium, heated from below is considered; a modified Darcy’s momentum equation and heat generation term used in energy equation. An autonomous system of fourth order differential equations have been deduced by using truncated Galerkin expansions. The asymptotic behavior can be stationary, periodic, or chaotic, depending upon the values of system parameters. It is found that a better combination of values of modulation parameters and scaled Rayleigh, internal Rayleigh numbers provides a way for chaos. Both a temperature and gravity modulation of the medium is to enhance or diminish the behavior of chaotic nature of the system. Due to high internal heat generation there is a chance to devolve a new kind of stable or unstable points in the system. Finally, heat transfer results are presented in terms of Nusselt number.

A comparative performance evaluation of the reversed Brayton cycle operated heat pump based on thermo-ecological criteria through many and multi objective approaches

Thermodynamic and ecological performance evaluation of the reversed Brayton cycle operated heat pump is analyzed in this work. Performance assessment of the heat pump is carried out through many-objective as well as multi-objective optimization. Maximization of coefficient of performance, ecological coefficient of performance, exergy efficiency and ecological function of the heat pump are considered simultaneously during many-objective optimization. Four operating variable of the heat pump which include temperature ratio, compressor pressure ratio, hot side and cold side heat exchanger effectiveness are investigated in the optimization study. The conflicting level between thermo-ecological behaviour of heat pump is explored through comparison between multi-objective (i.e. two-objective) and many-objective (i.e. four-objective) optimization results. Both, multi-objective and many-objective optimization problem of heat pump are solved using the heat transfer search algorithm and results are presented in the form of Pareto-optimal points. Further, the effect of the design variables and its sensitivity to thermo-ecological behaviour of the heat pump are also presented. Comparative results reveal that, 16.16–37.93% variation in the thermo-ecological behaviour of the heat pump is observed during many-objective optimization as compared to multi-objective optimization. Different decision making approaches are used to select a final optimal solution from Pareto optimal set of many-objective optimization. Finally, the optimization results are compared with the experimental results of the heat pump system. The comparative results revels that 10.95% deviation in co-efficient of performance and 12.3% deviation in exergy efficiency are observed between optimization and experimental results.

Effect of thickness and thermal conductivity of metal foams filled in a vertical channel – a numerical study

PurposeThis paper aims to discuss about the two-dimensional numerical simulations of fluid flow and heat transfer through high thermal conductivity metal foams filled in a vertical channel using the commercial software ANSYS FLUENT.Design/methodology/approachThe Darcy Extended Forchheirmer model is considered for the metal foam region to evaluate the flow characteristics and the local thermal non-equilibrium heat transfer model is considered for the heat transfer analysis; thus the resulting problem becomes conjugate heat transfer.FindingsResults obtained based on the present simulations are validated with the experimental results available in literature and the agreement was found to be good. Parametric studies reveal that the Nusselt number increases in the presence of porous medium with increasing thickness but the effect because of the change in thermal conductivity was found to be insignificant. The results of heat transfer for the metal foams filled in the vertical channel are compared with the clear channel in terms of Colburn j factor and performance factor.Practical implicationsThis paper serves as the current relevance in electronic cooling so as to open up more parametric and optimization studies to develop new class of materials for the enhancement of heat transfer.Originality/valueThe novelty of the present study is to quantify the effect of metal foam thermal conductivity and thickness on the performance of heat transfer and hydrodynamics of the vertical channel for an inlet velocity range of 0.03-3 m/s.

Preliminary Results for Flow Boiling Heat Transfer of R1233zd in Microchannels

The paper reports preliminary measurement results for flow boiling heat transfer of refrigerant R1233zd in parallel horizontal microchannels. The aluminum test section consists of two blocks and has 10 parallel channels with 1.0 mm in width, 1.5 mm in height and 440 mm in length. Five small thermocouple holes with diameter 0.6 mm and depth 20 mm were drilled at 10 locations along the channel on the two aluminum blocks, respectively. Local heat flux and channel surface temperature along the channel were obtained using the inverse heat conduction method [1] based on temperatures accurately measured by 100 thermocouples. Local saturation temperatures were obtained by assuming linear pressure distribution over the two-phase flow region along the channel. Experiments were conducted under different conditions. The refrigerant mass flow rate varied from 100 kg/m²s to 500 kg/ m²s. The heating water mass flow rate varied from 200 ml/min to 1300 ml/min. The heating water inlet temperature varied from 40℃ to 80℃. Preliminary results are presented and discussed.

Determination of interfacial heat transfer coefficient for the flow assisted mixed convection through brass wire mesh

In this work, a numerical investigation of Darcy‐Forchheimer mixed convection from a heated vertical flat plate embedded in a brass wire mesh porous medium is carried out to determine the coupled effects of flow and thermal diffusion. The numerical model consists of a two dimensional computational domain in which conjugate heat transfer analysis is performed to predict the hydrodynamic and thermal performance of the brass wire mesh in a vertical channel using Local Thermal Non-Equillibrium (LTNE) model. The novelty of the present study is to acquire the interfacial heat transfer coefficient, an as yet another challenging task, of the wire mesh porous medium so as to provide a quick and feasible solution to modeling of fluid flow and heat transfer through brass wire mesh porous media. The results of heat transfer through brass wire mesh are reported in terms of Colburn j factor, performance factor and are compared with other porous mediums available in literature. The present study not only opens up new vistas for more parametric studies but also provides practical and cost effective assessment to design new porous materials.

The comparison of a numerical and empirical calculation of thermal transmittance of ventilated facade with different heat-conductive connections

The energy efficient buildings are priority these days. The building energy efficiency requirements are increasing and thicker thermal insulation layer is used for building envelopes in order to achieve these high requirements. Ventilated facades are often used for the construction of public buildings, whose thermal insulating layers are crossed with connections made from various materials. Previously, steel and aluminium connections were used, whose influence on the heat transfer through the construction was high. When increasing the thickness of thermal insulation, the amount of connections should be increased, therefore the heat transfer will increase even more and such connections are no longer suitable for energy efficient buildings. Looking for less heat-conductive materials for connections, it has been observed that different calculation methods give different results of heat transfer, depending on characteristics of materials for thermal insulation and connections. In order to find out the reasons for these differences, calculations of ventilated walls with various connections were carried out. The heat transfer through the wall with connections crossing thermal insulating layer could be calculated in two ways. The first way is simpler and was often used earlier - this is the calculation of the heat transfer coefficient increase depending on type and geometry of connection using the formulas presented in the standards. A more precise calculation method is the calculation of the heat transfer coefficient of wall with connections using two - dimensional and three - dimensional heat transfer modelling computer software (heat flow analyses). The use of these two calculations methods shows the differences of results of the heat transfer through ventilated walls. For both calculations, the fragments of ventilated walls with connections made from aluminium alloy (λ - 160 W/(m∙K)), zinc coated steel (λ - 50 W/(m∙K)), stainless steel (λ - 17 W/(m∙K)) and glass fiber reinforced plastic (λ – 0,23 W/(m∙K)) were created. Calculations have shown that the heat transfer coefficient calculated for walls with zinc coated and stainless steel connections using different methods varies by 29 %, for walls with glass fiber reinforced plastic connections – by 0,4 %, and for walls with aluminium alloy connections - by 130 %. The main aim of this paper is the investigation of reasons of different calculations results according to numerical and empirical calculations methods. When the causes of discrepancies are found, empirical formulas can be corrected. DOI: http://dx.doi.org/10.5755/j01.sace.23.2.21204

Stability of PAM/PEI emulsified gels under HTHS conditions for water shut-off treatment

Emulsification has been widely used in the oil industry. The thermal stability of emulsified systems, such as emulsified gels or acids, is critical for the placement success of the emulsified system. Early separation of the system in a tubular well leads to corrosion. In the present study, the stability of emulsified polyacrylamide/polyethylenimine (PAM/PEI) gels is investigated in high-temperature and high-salinity (HTHS) environments. The influences of different parameters such as surfactant concentration, water–oil ratio, salinity, intensity of mixing and temperature on the droplet size and the emulsion thermal stability are studied. The droplet size of the PAM/PEI system, which is emulsified in diesel, decreases, and the separation time increases with increasing mixing speed, surfactant concentration, and salinity. The emulsions with smaller droplets are thermally stable compared with large droplets since the rate of droplet coalescence is low. The effect of emulsion stability on gelation kinetics and gel strength is investigated. The emulsification reduces both the gel strength and the gelation rate. The cross-linking (gelation) rate in the unemulsified PAM/PEI is almost ten times higher than that in the emulsified gels. This gelation process is slow in the case of stable emulsified PAM/PEI solution and is believed to be the result of limited heat and mass transfer. The emulsion stability achieved in this work adds to the current understanding of how to control emulsion stability through different parameters to withstand HTHS environments and how this stability influences the gelation process.

CHF experiments on the influence of inclination and gap size

CHF experiments for the downward-facing plate in a channel were performed varying inclination and channel gap size. A non-heating experimental methodology was employed to simulate the CHF condition based on the hydrodynamic analogy. Hydrogen gas generated on the cathode in an aqueous solution of sulfuric acid simulates vapor bubbles in the boiling system. Inclinations were varied from vertical to near downward-facing horizontal and the channel gap sizes were varied from 1 to 10 mm. In the open channel cases, the measured normalized CHF values showed similar trend with the existing heat transfer results. Considering the bubble shape variation depending on the inclination, the empirical correlation was developed. The influence of the bubble shape became prominent as the channel gap size decreased. However, for near horizontal inclination, the decreased buoyancy force became dominant so that the gap size effect was negligible.

In-line tube-bank heat exchangers: Arrays with various numbers of thermally participating tubes

This investigation presents numerical results for both laminar and turbulent flow and heat transfer for an in-line tube bank ranging in size from 1 to 20 tube rows over a Reynolds number range of 100–1000. Both the longitudinal and transverse pitches were fixed at 1.5D. It was demonstrated that the most useful heat transfer results were expressible as a total-tube-bank-averaged Nusselt number value, which is in contradiction to other investigations. For sufficiently lengthy tube banks, the existence of a fully developed regime characterized by an axially unchanging array-average Nusselt number was identified. It was found that the highest array-average heat transfer coefficients occurred in the initial portion of the tube bank, also in contradiction with information conveyed in some of the literature. A special case in which only a single tube in the array was thermally active was investigated in deference to experiments conducted under that condition. The present results obtained by numerical simulation compared favorably to existing experimental data.

Mixed Convection in a Rotating Concentric Annulus with a Porous Sleeve

Numerical solutions are presented for mixed convection in rotating concentric cylinders with a porous sleeve. The porous sleeve is press fitted to the inner surface of the outer cylinder. Whereas the inner cylinder is rotating at a constant speed, the outer cylinder remains stationary. The main objective of the present study was to investigate the flow pattern and temperature distribution as affected by the presence of the porous layer, the centrifugal force, and thermal buoyancy. A parametric study has been performed to investigate the effects of Peclet number, Rayleigh number, and Darcy number on the heat transfer results. Based on the results obtained, some implications for the industrial application of porous journal bearings can be made.

Effects of design parameters on a multi jet impinging heat transfer

The main focus of this work is a synergistic heat transfer analysis of the impinging multi jets. The geometry of the jet holes, arrangement of those holes, density of the jets through spanwise and streamwise directions and type of the confinement are the design parameters that affect the impingement characteristics. The numerical analysis is run by using commercial software ANSYS-CFX that uses computational fluid dynamics (CFD) codes. The average Nusselt number is extracted as the heat transfer result. It is concluded that, the heat transfer distribution under the densest-array jet impingement inside the one-exit-confined channel is behaved as the single jet because of the strong adjacent jet interference. The highest jet impingement heat transfer is achieved for the case using square jets. The effect of jet arrangement seems to be the weakest parameter on the heat transfer.

Avoiding Being Trapped in False Analogical Modeling of Composite Wall Thermal Resistance

Because Analogy is considered as a double-edged sword, thermal engineers should be cautious in analogical maneuvering between electrical and thermal domains in order not to be slipped into building misconceptions about thermal resistance concept. Composite wall thermal resistance (CWTR) modeling is one of the practical examples that illustrates the probability of misusing analogy. Heat transfer undergraduate textbooks coverage of CWTR suffers a lean towards “cookbook” coverage that reports concise statements that lack deep clarification and illustration. Transparent Thinking Approach (TTA) is employed to present a detailed calculation and illustration of a typical CWTR modeling based on isothermal and adiabatic assumptions. The calculation of a typical CWTR for different values of wall thermal conductivities shows that the difference in parallel walls thermal conductivity is creating a large discrepancy that may reach 80% between heat flows calculated based on isothermal and adiabatic assumptions. It is found that for a series-parallel arrangement of composite walls with high difference in parallel wall thermal conductivity values, the true value of heat flow is bracketed between the isothermal and adiabatic heat flow values. The transparent way of presenting CWTR modeling can be readily included in any standard heat transfer textbook and result in greatly enhancing CWTR modeling coverage.

Large eddy simulation analysis of the heat transfer enhancement using self-oscillating fluidic oscillators

Using an unsteady jet created by the fluidic oscillator represents many opportunities to enhance the heat removal performance of impinging jet. Numerical simulations were conducted to compare the heat transfer performance of two fluid oscillators and a direct jet with unsteady Reynolds-averaged Navier-Stokes (RANS) and large eddy simulation (LES) turbulence models implemented. Time-resolved and time-averaged flow fields and heat transfer results were presented. Simulation results showed that the fluid dynamic and heat transfer performance predicted by the RANS and LES models matched well for different Reynolds numbers.The self-oscillating impinging jets were beneficial in enhancing the heat removal performance by increasing the average Nusselt numbers and covering a larger impinging cooling surface. The wide impinging zone and significant heat transfer enhancement suggested it was promising to implement self-oscillating impinging jet concept into existing cooling technologies, such as multi-phase cooling and thin film cooling, for the next-generation high-performance heat exchangers.

Effect of inclination on heat transfer and flow regimes in large flattened-tube steam condensers

An experimental study of convective steam condensation inside a large, inclined, flattened tube used in air-cooled condensers for power plants is presented. This is the fourth of a four-part group of papers. The first three parts (Kang et al., 2017; Davies et al., 2017, 2018) published in the same journal present the facility, pressure drop, void fraction, flow regime and heat transfer results, while this study presents the effects of inclination on heat transfer and flow regimes.The condenser is a flattened steel tube with brazed aluminum fins. The full tube has dimensions 10.72 m × 214 mm × 18 mm. The condenser tube is cut in half lengthwise and covered with a polycarbonate window to perform visualization simultaneously with the heat transfer measurements. The steam is condensed at atmospheric pressure, and cooled by air at a uniform velocity profile. The angle of inclination is varied from horizontal (0°) to 75° downward. Condenser performance is also predicted with a model.The majority of the condenser is found to be in the stratified flow regime for all inclinations tested, with only a short annular section at the inlet of the condenser. The tubes inclined greater than 60° are also found to have stratified-wavy flow near the condenser outlet. Overall condenser U is found to increase with increasing downward inclination angle of the condenser, with a maximum increase of approximately 4% at 75° inclination. This improvement is found to be the result of improved drainage and increased void fraction near the condenser outlet. Mean steam-side heat transfer coefficient1HTC – Heat transfer coefficient1 (HTC) is found to remain constant along the tube, and for the entire condenser, with changes in tube inclination angle. Commonly-used inclined condensation HTC correlations are found to underpredict the magnitude of the experimentally-determined steam-side HTC.

Heat transfer optimization of a fully solid state micro-unit regeneration magnetic refrigerator

This paper introduced and numerically investigated a fully solid state magnetic refrigerator based on a micro-unit regeneration cycle and Peltier elements, in which Peltier elements were used to enhance heat transfer and heat regeneration. Due to the limitation of heat transfer rate under solid state, topology optimization was used to improve the heat transfer structure of the magnetic refrigerator. The heat transfer behaviors of the three heat transfer configurations were tested and compared experimentally: Gd-only, parallel sheets and topology optimization structure. The cycle performance before/after optimization and without/with Peltier elements was then carefully compared and discussed using a validated simulation model. The results showed that the performance of the fully solid state magnetic refrigerator was greatly improved after heat transfer optimization. The improvement was more significant when operating with Peltier elements. The heat transfer optimization approaches, results and conclusions can serve as guidelines for solid state MR design and optimization.

A Comparison of Experimental and Computational Heat Transfer Results for a Leading Edge Impingement System

Leading edge impingement systems are increasingly being used for high pressure turbine blades in gas turbine engines, in regions where very high heat loads are encountered. The flow structure in such systems can be very complex and high resolution experimental data is required for engine-realistic systems to enable code validation and optimal design. This paper presents spatially resolved heat transfer distributions for an engine-realistic impingement system for multiple different hole geometries, with jet Reynolds numbers in the range of 13,000–22,000. Following this, Reynolds-averaged Navier-Stokes computational fluid dynamics simulations are compared to the experimental data. The experimental results show variation in heat transfer distributions for different geometries, however average levels are primarily dependent on jet Reynolds number. The computational simulations match the shape of the distributions well however with a consistent over-prediction of around 10% in heat transfer levels.

Enhancement of heat transfer via corona discharge by using needle-mesh and needle-fin electrodes

As a potential alternative of traditional heat transfer enhancement technology, ionic wind has drawn great attention. In this study, a direct comparison of the ionic wind devices with needle-mesh and needle-fin electrodes is carried out experimentally. The ionic characteristics and cooling performances of two electrode types here are tested and quantitatively analyzed by calculating the empirical correlations defining the relationship between electrohydrodynamics and heat transfer. The results indicate that the electrode gap d and the number of needle electrodes N are the main factors of ionic characteristics such as current, velocity and efficiency. And the results of heat transfer show that the needle-fin configuration has a superior performance, which has made a temperature drop from 54.5 °C to 39.1 °C with the low power consumption of 0.85 W. An empirical correlation for calculating the Nu-Re relation in the case is developed, which reflects that the ionic wind is a fluid with thinner boundary layer and stronger convection. In addition, the heat transferred by the fins in the ionic wind device is more than twice that of the convection on the plate surface. Finally, the feasibility and advantages of using fins as collector electrodes are confirmed.

Thermal-hydraulic performance of printed circuit heat exchangers with zigzag flow channels

Printed circuit heat exchanger (PCHE) is one of the leading candidates to be employed in advanced nuclear reactors and next generation concentrated solar power applications due to its compactness and capability for high-temperature, high-pressure applications with high effectiveness. In the current study, thermal-hydraulic performance of a zigzag-channel PCHE with high-pressure, high-temperature helium on both the hot and cold sides was simulated using a computational fluid dynamics (CFD) software package STAR-CCM + . Comparisons between the experimental data and CFD simulation results showed good agreement with some discrepancies in the pressure drop and heat transfer results. Local thermal-hydraulic performance analyses indicated that a fully-developed flow condition was not observed in the PCHE, mainly due to the nature of the zigzag channels, leading to periodic flow disturbance at each of the zigzag bends. It was also found that the local and global heat transfer coefficients were considerably different in the PCHE. Furthermore, thermal boundary conditions showed that the fluid temperatures and heat fluxes were not uniform along the azimuthal direction of a cross section of the flow channel and that the helium temperature distribution for each segment along the flow direction presented a wavy profile. However, the distribution of the helium bulk temperature along the flow direction was approximately linear. For the heat flux distributions, although they were significantly different at different segments, the trend of the heat flux for each segment along the fluid flow direction was similar. Finally, effects of several parameters on the thermal-hydraulic performance of the PCHE were investigated, including the fluid and solid thermophysical properties, radius of curvature at zigzag bends, channel configuration, channel length pitch in the flow direction, and zigzag pitch angle. No considerable enhancement in the Nusselt numbers was observed when the zigzag pitch angles were greater than 30°.

Experimental investigation of heat transfer from different pin fin in a rectangular channel

ABSTRACTIn this study, the effect of both hexagonal pin fins (HPFs) and cylindrical pin fins (CPFs) into the rectangular channel on heat transfer augmentation, Nusselt number and friction factor were experimentally investigated. In planning of the experiments, different Reynolds number, pin fin array, pin fin geometry and the ratio of the distance between pin fin spacing (s) to the pin fin hydraulic diameter (s/Dh) were chosen as the design parameters. Air was used as the fluid. The Reynolds number, based on the channel hydraulic diameter of the rectangular channel, was varied from 3188 to 19531. In the experiments, the heating plate was made of stainless steel foil. The foil was electrically heated by means of a high current DC power supply to provide a constantly heated flux surface. The heat transfer results were obtained using the infrared thermal imaging technique. The heat transfer results of the hexagonal pin fins (HPFs) and cylindrical pin fins (CPFs) are compared with those of a smooth plate. Best heat transfer performance was obtained with the hexagonal pin fins. The maximum thermal performance factor ((Ƞ), was obtained as Re = 3188, staggered array, s/Dh = 0, Ƞ = 2.28.

Numerical investigation of heat transfer by an impinging jet using alumina–water nanofluid

This article reports numerical investigations of turbulent flow field and heat transfer performance of water-alumina nanofluid jet impingement. The jet impinges over a flat, circular heated surface from a circular pipe of diameter . The influence of jet Reynolds number (1,200 ≤ ≤ 40,000), jet-outlet-to-target wall distance (2 ≤ ≤ 12.8), and particle volumetric concentration (0 ≤ φ ≤ 10%) on the fluid flow and heat transfer are examined. The results of fluid flow and heat transfer are compared with the published experimental data. It is found that the heat transfer increases with increasing values of the particle volumetric concentration while the flow field and turbulent kinetic energy distribution remain unchanged with the inclusion of nanoparticles. About 28% enhancement in stagnation Nusselt number is observed at low values of nanoparticle volumetric concentration (φ = 0.1%) and in the Reynolds number range of 29,000–40,000. For jet-outlet-to-target wall spacing values used in the current study, the local Nusselt number distributions vary according to Nu ∝ . Numerical simulations are performed using Reynolds-averaged Navier–Stokes method with the shear stress transport k-ω model in commercial CFD code ANSYS-CFX.