High Local Heat Transfer Coefficient Research Articles

Heat transfer enhancement of a slot-confined and submerged impinging jet utilizing lamina-shaped CoFe3O2/water nanofluid

Designing a cooling system is crucial for the thermal management of many different types of energy applications, such as fuel cells, solar panels, electronic cooling, and many more. A higher local heat transfer coefficient is attained by impinging jets, making them a viable cooling option. This study investigates a two-dimensional numerical study into the turbulent convective heat transfer in a confined slot and submerged impinging jet by using water and a nanofluid for Reynolds numbers between 6000 and 24000. The nanofluid of lamina-shaped CoFe3O2/water has been studied, with the volume concentration of nanoparticles ranging from 2.0% to 4.0%. Using a finite volume technique based on the SIMPLE algorithm, the governing momentum, continuity, and energy equations are solved. A presentation and discussion of the influence of the nanoparticle's volume fraction and the Reynolds number on the flow and thermal properties are provided. Increasing the volume fraction of nanoparticles is shown to enhance the Nusselt number and the Darcy friction factor. Entropy generation increases with the increase of the Reynolds number for all working fluids.

CFD model of acceleration of thermal-hydrodynamic processes in solar air collectors

Recent studies on increasing the thermal-hydrodynamic efficiency of solar air collectors have been carried out on the installation of barriers of various shapes on the surface of the absorber, and this method ensures a significant increase in the energy efficiency of the collector. The transfer of the air flow washing the surface of the absorber from a laminar flow state to an accelerated lumped air flow is carried out by installing obstacles. Installation of barriers is the main factor in increasing the heat transfer in solar air collectors and prolongs the time of air flow in the collector. The barrier solar air collector has a high local heat transfer coefficient, and the Nusselt number value is up to 3.5 times higher than that of the flat plate solar air collector. Also, this article presents the results of CFD modeling of the air flow in the solar air collector, the results of which can be used in the theoretical research of the solar air collector.

Enhancing Jet Array Heat Transfer: Review of Geometric Features of Nozzle and Target Plates

Adequate cooling of electronic components and packages is an ever-increasing challenge as devices become more powerful, compact, and portable. Highly integrated and high-performance solutions are sought across a wide range of industrial and commercial sectors, requiring new methods and means of cooling beyond the traditional fan-fin heat sink. Impinging jets, when arranged in arrays, can achieve very high local and surface-averaged heat transfer coefficients whilst providing sufficient surface area coverage for a broad range of electronic devices. As such, they are a high-potential candidate for next generation heat exchangers for cooling high heat flux electronics. This review article focusses primarily on the means by which the heat transfer of impinging jet arrays can be enhanced by straight-forward alterations of the geometric features of the jet orifices, nozzle plate, and impingement surface, as compared with the baseline case of round jets impinging on flat surfaces. From non-circular jets, to patterned micro-roughness, the review aims to collate and synthesize the state-of-the-art, highlighting advantages and pitfalls, to aid thermal engineers in designing high performance jet impingement heat exchangers.

Utilization of wavy porous layer, magnetic field and hybrid nanofluid with slot jet impingement on the cooling performance of conductive panel

PurposeThe purpose of this study is to analyze the impacts of combined utilization of multi-jet impinging cooling of nanofluids with magnetic field and porous layer on the cooling performance, as effective cooling with impinging jets are obtained for various energy systems, including photovoltaic panels, electronic cooling and many other convective heat transfer applications.Design/methodology/approachFinite element method is used to explore the magnetic field effects with the inclusion of porous layer on the cooling performance efficiency of slot nanojet impingement system. Impacts of pertinent parameters such as Reynolds number (Re between 250 and 1,000), strength of magnetic field (Ha between 0 and 30), permeability of the porous layer (Da between 0.001 and 0.1) on the cooling performance for flat and wavy surface configurations are explored.FindingsIt is observed that the average Nusselt number (Nu) rises by about 17% and 20.4% for flat and wavy configuration while temperature drop of 4 K is obtained when Re is increased to 1,000 from 250. By using magnetic field at the highest strength, the average Nu rises by about 29% and 7% for flat and wavy cases. Porous layer permeability is an effective way of controlling the cooling performance while up to 44.5% variations in the average Nu is obtained by varying its value. An optimization routine is used to achieve the highest cooling rate while the optimum parameter set is obtained as (Re, Ha, Da, γ, sx) = (1,000, 30, 0.07558, 86.28, 2.585) for flat surface and (Re, Ha, Da, γ, sx) = (1,000, 30, 0.07558, 71.85, 2.329) for wavy surface configurations.Originality/valueIn thermal systems, cooling system design is important for thermal management of various energy systems, including fuel cells, photovoltaic panels, electronic cooling and many others. Impinging jets are considered as effective way of cooling because of its ability to give higher local heat transfer coefficients. This paper offers novel control tools, such as magnetic field, installation of porous layer and hybrid nano-liquid utilization for control of cooling performance with multiple impinging jets.

Study on flow and heat transfer characteristics of Encapsulated Phase Change Material (EPCM) slurry in Double-Pipe Heat Exchanger

Microencapsulated phase change material (MPCM) slurry as a working fluid was numerically investigated for thermal energy conservation systems due to its high heat capacity associated with the PCM. A mathematical model of a double pipe heat exchanger was developed to conduct the numerical analysis based on the Eulerian-Eulerian flow model. During simulation, it was considered that MPCM slurry flows through the inside pipe of the heat exchanger, whereas high-temperature plain water flows at the annulus side of the heat exchanger. The accuracy of the model was verified with experimental data and showed a good agreement. The effect of particle concentration, Reynolds number, and particle diameter was considered to investigate the energy transport characteristics of the MPCM slurry. The results showed that particle concentration significantly affects the heat transfer performance of MPCM slurry, and it promotes pressure drop due to an increase in viscosity. The highest average heat transfer coefficient of the slurry showed at 15% volume fraction; it was also significantly affected by the particle diameter greater than 100 µm. The local heat transfer coefficient of water and MPCM slurries were compared at a Reynolds number of 6838, and PCM-based HTFs showed a higher local heat transfer coefficient. Furthermore, a ratio of h/ΔP was presented to study the energy transport performance of MPCM slurry at different particle concentrations and compared with plain water, and it was found out that 10% volume fraction showed better performance as HTF (Heat Transfer Fluid).

Detailed heat transfer investigation of an impingement jet array with large jet-to-jet distance

The heat transfer performance of a large jet-to-jet distance impingement configuration with a jet-to-jet spacing X/D = Y/D = 10 was experimentally studied for turbomachinery applications. Narrowband thermochromic liquid crystals (TLCs) were used to record the target wall temperature. Pressure taps were set along the experimental rig to measure the pressure loss. Improved measurement methods with multiple TLCs and a ramp heater were studied. The work was done for different Reynolds numbers (Re = 15,000–35,000), different jet to target plate distances (H/D = 3–5) and different cross-flow schemes. The results from the improved methods show good agreement with the basic measurement method. It confirms that the single TLC, step heater method is sufficient for the current experimental study. The improved methods are confirmed to be reliable and are suitable for complex applications with extremely large heat transfer variations over the measured surface, or extremely high local heat transfer coefficients. Experimental results indicate that a smaller jet to target plate distance and a smaller cross-flow scheme can provide a better heat transfer performance in the studied range. When compared to the impingement configuration with X/D = Y/D = 5, the results show that a larger jet-to-jet distance configuration provides a higher overall heat transfer when the mass flow rate is kept constant.

Performance analysis and design optimization of heat pipe sink with a variable height fin array under natural convection

To address the cooling problems in electronic equipment with high heat flux but small area under natural convection, a heat pipe sink with a novel fin array of unequal and continuously changing heights is proposed to improve heat transfer by reducing flow resistance. By means of three-dimensional numerical calculations, the comparison of heat transfer performance of heat pipe sinks with different variable-height fin arrays is performed. The results show that the fin arrays with larger height difference have lower flow resistance and higher local heat transfer coefficient. Parameter studies show that increasing the fin spacing (s) and the maximum fin height difference (p) can greatly reduce the material cost per unit power, Mtot, but their effects on the total thermal resistance, Rtot, are more complex. A multi-objective optimization analysis is further conducted based on the response surface method with consideration of minimum Rtot and minimum Mtot as objectives. Additionally, the optimization technique was integrated with the Non-dominated Sorting Genetic Algorithm II (NSGAII) and the Technique for Order Preference by Similarity to Ideal Solution (TOPSIS) in order to improve the search speed and accuracy of Pareto solutions. By analyzing the Pareto solutions obtained, it is found that the influence of p and s on the system performance is related rather than independent.

Comparison between cooling strategies for power electronic devices: fractal mini-channels and arrays of impinging submerged jets

Power electronic devices like Insulated Gate Bipolar Transistors (IGBTs) and diodes are often characterized by power densities and dimensions that could result in very high heat flux densities. In order to guarantee the expected performance and lifetime for these components, dedicated active cooling devices are usually adopted. In the present paper, the comparison between two different cooling strategies for power electronics is presented: fractal-channel design and submerged impinging jets. Each cooling strategy is tested on two different geometrical configurations. Water is used as coolant in all cases. Assessment of the considered cooling methods is done through application of the selected configuration in a simplified system composed by a rectangular chip (heat source) separated from the coolant by a solid block.Three-dimensional conjugated heat transfer simulations are performed by using RANS solver implemented in OpenFOAM and two-equations turbulence models, resolving also the viscous sublayer. Numerical results allow to compare the cooling strategies in terms of maximum chip temperature, overall chip-to-coolant thermal resistance, and pumping power required. In summary, the fractal-channel design shows limitations in guaranteeing low chip temperatures at an affordable pumping power. The submerged impinging jets approach shows very high local heat transfer coefficient by which it is possible to tailor the cooling effect on specific hot spots.

A study of the vortex-induced lateral vibration and heat transfer characteristics of elastic supported single tubes with different cross-sectional shapes

In this paper, the vortex-induced vibrational characteristics and heat transfer mechanisms of three tubes with different cross-sectional shapes are numerically studied. The results show that the characteristics of vortex-induced vibration of the vertical elliptical tube and circular tube are obvious. At low flow velocities, the vortex shedding force can affect the vibrational characteristics of horizontal elliptical tube, but the effect is not obvious with increasing flow velocity. When the frequency is locked, the vibrational amplitude of the vertical elliptical tube is the largest, followed in descending order by that of the circular tube, and that of the horizontal elliptical tube. The local heat transfer coefficients of tube at different vibration time are different. High local heat transfer coefficients are produced when vortices form on local walls or—when the relative velocity between the fluid and the heat transfer wall is relatively large. The effects of vortex-induced vibration enhancement heat transfer of these three kinds of tubes are different. The vertical elliptical tube can enhance the heat transfer by 28.6%, the circular tube can enhance it by 21.3%, and the horizontal elliptical tube can only enhance it by 3.7%.

Separation in condensers as a way to improve efficiency

This paper introduces the concept of separation of two-phase flow in condensers and discusses its possible application of enhancing the heat transfer performance by capitalizing on the high local heat transfer coefficient of vapor flow. The benefit of vapor–liquid refrigerant separation and the reason why it will improve the condenser performance are explained. Numerical studies are performed on an R-134a microchannel condenser. Model predicts that at the same mass flow rate, the exit temperature is lower by 1.3 K in the separation condenser than in the baseline condenser while the difference of pressure drop remains within 2%. 6.1% more flow rate of condensate is predicted in the separation condenser as another comparison criterion. In addition, the trade-off between high quality and low mass flux for the vapor path downstream of the separation header is investigated by the model and results are presented. Modeling is conducted with pre-assumed separation efficiency in the header. The real value requires further investigation.

Numerical simulation of 3D hollow-fiber vacuum membrane distillation by computational fluid dynamics

Computational fluid dynamics (CFD) was employed for modeling and simulation of the heat and mass transfer processes in hollow-fiber vacuum membrane distillation (VMD) under laminar flow conditions. A three-dimensional VMD model was first developed and validated by coupling the latent heat with the energy conservation equations and experimental data. Then it was used to analyze the effects of operating conditions and module dimensions on local temperatures, heat transfer coefficients, temperature polarization coefficients, heat and mass fluxes and total thermal efficiency in an operation of the feed flowing in the lumen of the fibers and the shell in vacuum. Thermal efficiency varied with feed temperature and feed velocity. Temperature polarization became more significant at high feed temperature and low feed velocity. Mass transfer was controlled by the heat transfer in the feed boundary. Local heat and mass fluxes decreased along the fiber length because of high local heat transfer coefficient in the inlet region and thin thermal boundary. More importantly, total thermal efficiency decreased with increasing module length, thus a short module was better used for high efficiency of VMD.

Investigation on heat transfer characteristics of R134a spray cooling

Due to its high local heat transfer coefficients, spray cooling is a vital technique for thermal management of high heat flux electronic devices. In this study, an experimental system using R134a as the working fluid was designed and built to study the heat transfer characteristic of spray cooling. The influences of the liquid flow rate, the vapor phase mass fraction and the liquid subcooling of the two phase R134a spray cooling on the heat transfer performances were discussed. The experimental results show that the heat transfer coefficient increased with the increasing liquid flow rate when the liquid fluid rate was less than 1.6L/min. However, the enhancement on heat transfer coefficient owing to the increase of liquid flow rate was insignificant while the flow rate was higher than 1.6L/min. A maximum surface heat flux of 325.99W/cm2 was achieved using R134a spray cooling while the temperature of the test surface was about 42.4°C at the flow rate of 2.15L/min.

Forced convective heat transfer of slush nitrogen in a horizontal pipe

In the present study, the flow and heat transfer characteristics of slush nitrogen in a horizontal pipe of 10.0mm in diameter were investigated experimentally and theoretically. The flow velocity was in the range of 0.9–3.4m/s, and the solid volume fraction was up to 30.0%, and the slush Reynolds number ranged from 1.89×104 to 1.01×105. The Eulerian–Eulerian multiphase model incorporated with the kinetic theory of granular flow was employed to investigate the forced convective heat transfer of slush nitrogen by taking into account of the heat and mass transfer between the solid and liquid phases. It was found that the pressure drop of slush nitrogen was higher than that of subcooled liquid nitrogen and increased with the solid volume fraction due to the existence of solid particles, and it showed different dependences on the flow velocity in the heterogeneous and pseudo-homogenous flows. Due to the latent heat involved in the solid–liquid phase change, the local heat transfer coefficients of slush nitrogen were higher than that of subcooled liquid nitrogen. The heat transfer between solid and liquid phases was enhanced as the solid volume fraction increased, leading to higher local heat transfer coefficient and lower and more stable fluid temperature of slush nitrogen. Although the dependences of the local heat transfer coefficient on the velocity and solid volume fraction were similar in both the homogeneous and heterogeneous flows, the distribution of the fluid temperature on the cross-section was found to vary with the flow patterns because of the non-uniform distribution of solid particles. The heat transfer correlation was proposed based on the experimental results for predicting the heat transfer performance of the heterogeneous and pseudo-homogenous slush nitrogen flows with various solid volume fractions.

Local heat transfer coefficient measurement through a visibly-transparent heater under jet-impingement cooling

The investigation of transient and steady jet-impingement requires highly-sensitive local and instantaneous temperature measurements in order to accurately establish the characteristically high heat transfer coefficients. A structured method is proposed, wherein IR thermography measurements are taken from a heater that is visibly-transparent yet infrared-opaque. The heater is realized as a transparent conductive coating, Indium Tin Oxide, applied to an IR window, and allows the application of simultaneous optical and thermal measurement. Here, high-speed photography is synchronized to the IR thermography for the evaluation of the temporal sensitivity and response of the heater. For increased accuracy, the IR emissivity of the heater is established at temperatures far above the ambient through a dedicated experimental setup, and thermal-emittance subtraction is employed.Results are presented for the local temperature distribution in several steady and transient cases of water jet impingement in the transitional and weakly-turbulent region (Reynolds number 2400 to 3800): the start-up and steady state of a vertical submerged jet; as well as the steady-state, pulsed-flow and intermittent-flow of a horizontal free-jet. No time-delay was found in the heater’s thermal response, and even highly-transient phenomena, such as a droplet impinging on the liquid-film, a bubble flowing down the heater or large scale dryout/re-flooding can be clearly captured, optically and thermally. However, the thermal response is dampened by the substrate (IR window), and therefore the heat transfer coefficient is only calculated in steady state cases, where noise levels are further reduced by time-averaging. For the values of the local heat transfer coefficient, good agreement with the literature is found, in both free and submerged-jets.Due to a high sensitivity of the method, evidence of a local heat transfer peak is found at a distance of around 0.5 nozzle diameters from the stagnation point and some asymmetry is found under the influence of gravity. In the case of a free jet impinging on a vertical heater the measurement method is extended to allow local temperature measurement from both sides of the liquid-film flowing down the heater, by employing an IR mirror. From these measurements, a truly localized heat transfer coefficient is found that accounts for the heating-up of the liquid as it flows down the heater and develops into a falling liquid film, resulting in higher local heat transfer coefficients, beyond 3 diameters downstream from the stagnation point.Finally, an error analysis is conducted and some quantative recommendations are made regarding the range of validity of this method, in terms of the minimal temperature difference over which the heat transfer coefficient can reliably be found.

Numerical simulation of thermofluid characteristics of two-phase slug flow in microchannels

A fundamental study of heat transfer characteristics of two-phase slug flow in microchannels is carried out employing the Volume-of-Fluid (VOF) method. Despite of the fact that numerical simulations of two-phase flows in microchannels have been attempted by many investigators, most efforts seem to have failed in correctly capturing the flow physics, especially those pertaining to the slug flow regime characteristics. The presence of a thin liquid film in the order of 10 μm around the bubble is a contributing factor to the above difficulty. Typically, liquid films have a significant effect on the flow field and heat transfer characteristics. In the simulations reported in this paper, the film is successfully captured and a very high local convective heat transfer coefficient is observed in the film region. A strong coupling between the conductive heat transfer in the solid wall and the convective heat transfer in the flow field is observed and characterized. Results showed that unsteady heat transfer through the solid wall in the axial direction is comparable to that in the radial direction. Results also showed that a fully developed condition could be achieved fairly quickly compared to single-phase flows. The fully developed condition is defined based on the Peclet number ( Pe) and a dimensionless length of the liquid slug. Local and time-averaged Nusselt numbers for slug flows are reported for the first time. It was found that significant improvements in the heat transfer coefficient could be achieved by short slugs where the Nusselt number was found to be 610% higher than in single-phase flows. The study revealed new findings related to slug flow heat transfer in microchannels with constant wall heat flux.

Numerical investigations of circular tube fitted with helical screw-tape inserts from the viewpoint of field synergy principle

3-D numerical simulations are performed for heat transfer and fluid flow characteristics of the tubes fitted with helical screw-tape inserts. The comparison between the RNG k-epsilon turbulence model and the SST k-omega turbulence model in prediction of swirling flow shows that the latter has better qualitative and quantitative agreements with the experimental results than the former. The simulation results show that the novel helical inserts with right and left twists alternately have better heat transfer performance than that of the helical inserts with uniformly right twists. The simulation results are analyzed from the viewpoint of field synergy principle, it is found that the principle could explain well the mechanism of heat transfer enhancement of the helical twist inserts. The smaller the mean intersection angle between the temperature gradient and velocity vector, the larger the Nusselt number; in addition, the smaller local intersection angle also implies the higher local heat transfer coefficient.

Sidewall Effects on Heat Transfer Coefficient in a Narrow Impingement Channel

This paper examines the local and averaged effects of channel height in the presence of sidewalls on heat transfer coefficients, with special attention given to the sidewall behavior. High resolution local heat transfer coefficient distributions on target and sidewall surfaces were computed using temperature sensitive paint, recorded via a scientific-grade charge-coupled device camera, and compared with available literature. This is important, as many of the major correlations related to impingement channels are directly applicable to wide arrays of jets, for which the influence of the presence of sidewalls is minimal. Streamwise pressure distributions were recorded and used to explain heat transfer trends and to determine the thermal effectiveness of each channel. Results are presented for average jet-based Reynolds numbers between 17,000 and 45,000. All experiments were carried out on a large-scale single-row 15-hole impingement channel (with an X/D of 5, a Y/D of 4, and a Z/D of 1, 3 and 5). It was observed that available correlations accurately predict the target surface heat transfer coefficients when the influence of the sidewall is minimal, typically at larger channel heights and lower Reynolds numbers. Smaller channel heights tend to outperform larger channels, when considering thermal effectiveness and channel-averaged heat transfer rates, due to the increased heat transfer on the channel sidewalls.

Development of modular helium-cooled divertor for DEMO based on the multi-jet impingement (HEMJ) concept: Experimental validation of thermal performance

A modular helium-cooled divertor design based on the multi-jet impingement concept (HEMJ) that is capable of accommodating a surface heat flux of at least 10 MW/m 2 has been developed at the Forschungszentrum Karlsruhe [P. Norajitra, et al., He-cooled divertor for DEMO: experimental verification of the conceptual modular design, Proceedings of the ISFNT-7, May 22–27, 2005, Tokyo, Japan. Fusion Engineering and Design, 81 (1–7) (2006) 341–346]. Thermal-hydraulic simulations used in the original design optimization process predicted extremely high local heat transfer coefficients (∼50 kW/(m 2 K)) [R. Kruessmann, Parametric study to evaluate the design options of the HEMJ divertor cooling concept by CFD simulations (Task TW4-TRP-001). Internal Report, Forschungszentrum Karlsruhe, July 2006]. This prompted an experimental investigation to validate the numerical results. A one-to-one scale test section of the HEMJ divertor reference geometry has undergone testing in an air flow loop under a wide range of operating conditions that include the Reynolds number of the actual helium-cooled HEMJ design. The temperature distribution and local heat transfer coefficients have been measured and compared to numerical predictions of the ANSYS FLUENT ® CFD software package [ANSYS FLUENT ® 6.2.16, ANSYS FLUENT Inc., 2005]. The experimental data closely match the numerical predictions. This comparison provides confidence in using the numerical model for the design of the HEMJ divertor, as well as other gas-cooled high heat flux components. Future experiments will be performed using the same test module with helium cooling at prototypical Reynolds numbers.

Visualization of convective boiling heat transfer in single microchannels with different shaped cross-sections

Convective boiling in transparent single microchannels with similar hydraulic diameters but different shaped cross-sections was visualized, along with simultaneous measurement of the local heat transfer coefficient. Two types of microchannels were tested: a circular Pyrex glass microtube (210 μm inner diameter) and a square Pyrex glass microchannel (214 μm hydraulic diameter). A 100-nm-thick semi-transparent ITO/Ag thin film sputtered on the outer wall of the microchannel was used for direct joule heating of the microchannel. The flow field visualization showed semi-periodic variation in the flow patterns in both the square and circular microchannels. Such variation was because the confined space limited the bubble growth in the radial direction. In the square microchannel, both the number of nucleation bubbles and the local heat transfer coefficient increased with decreasing vapor quality. The corners acted as active nucleation cavities, leading to the higher local heat transfer coefficient. In contrast, lack of cavities in the smooth glass circular microchannel yielded a relatively smaller heat transfer coefficient at lower vapor quality. Finally, the heat transfer coefficient was higher for the square microchannel because corners in the square microchannel acted as effective active nucleation sites.

Glove thermal insulation: local heat transfer measures and relevance.

When exposed to cold, the hands need to be protected against heat loss not only in order to reduce thermal discomfort, but also to keep their efficiency. Although gloves are usually the most common protection, their thermal insulation is generally unknown. The aim of this study was to measure the heat losses from a gloved hand with a special interest in local variations. Using a calorimetric hand placed in a cold box, several types of gloves were tested. The results indicated that depending on the glove and on the area covered the heat loss reduction may vary from almost 60% to 90%. When the least efficient pair of gloves was excluded, heat exchange coefficients varied from 1.8 to 4.8 W/m2 per degrees C for the palm and from 4.2 to 6.2 W/m2 per degrees C for the back of the hand. The three medium fingers seemed to be equally treated, with a heat exchange coefficient variation of 6.3-9.0 W/m2 per degrees C. The thumb and the little finger, which require better insulation, exhibited higher local heat transfer coefficients of 8.3-12.7 W/m2 per degrees C. Some practical aspects are evoked.