Local Heat Transfer Distribution Research Articles

Local Heat Transfer Distribution On Thin Metal Foil Impinged by Array of Free Surface Jets

Abstract The present study focuses on the measurement of the local heat transfer distribution of a smooth flat plate impinged by an array of free surface jets on a thin metal foil. Local Nusselt number distributions are measured for a fixed jet diameter (d = 3 mm). The jet arrays consist of perfectly round apertures arranged in a square pattern, with a uniform spacing of 4d between adjacent jets in both the streamwise and spanwise direction. A wide range of Reynolds numbers varying from 1000 to 12500 are covered in this study. The nozzle to plate spacing (z/d) is varied between 1 to 10. The effect of the Reynolds number and nozzle to plate spacing on the local and spanwise average Nusselt number distributions are studied. The local Nusselt number exhibits an increase with nozzle-to-plate spacing within the low Reynolds number range (Re = 1000 to 2500). However, for Reynolds numbers exceeding 2500, the influence of nozzle-to-plate spacing on Nusselt number distributions remains negligible up to a nozzle-to-plate distance (z/d) of 5. Beyond this point, there is a gradual decrease in the Nusselt number value. The Nusselt number value gradually decreases beyond z/d of 5. At a Reynolds number of 1500, the Nusselt number increases by 71% for z/d = 10 in comparison to z/d = 1. Empirical correlations for local and spanwise average Nusselt number are proposed which satisfactory predict the local as well as spanwise average Nusselt number distributions.

Changes in boiling regime and heat transfer effectiveness during water droplet collision with a superheated wall

We experimentally investigated changes in boiling regime and heat transfer effectiveness when single droplets collided with superheated substrates at various initial temperatures. Saturated water droplets colliding at constant speeds were studied over a range of initial substrate temperatures that spanned all boiling regimes from nucleate through transition to film boiling. Space- and time-resolved infrared thermometry and convectional shadowgraph imaging were used to precisely derive local heat transfer distributions and detect characteristic boiling regimes. The results confirmed the existence of the typical regions of a boiling curve: nucleate, transition, and film. The transition boiling regime could be subdivided into three sub-regimes—bubbly, oscillating, and fingering—during which the heat transfer effectiveness (THE) varied in a non-linear manner, exhibiting local minima and maxima. To facilitate interpretation of non-linear variations in HTE during transition boiling, the basic physical parameters of local heat flux, contact area, and residence time were quantitatively measured using obtained thermometry images. In the transition boiling regime, HTE first rapidly decreased during bubbly boiling principally because of a substantial decrease in the droplet residence time. And THE increased during oscillating boiling because of contact area expansion and increased heat flux. Finally, THE decreased during fingering boiling due to substantial reduction in the area fraction of liquid contact. When liquid-solid contact was completely prohibited, stable film boiling was stable. Thus, the dynamic Leidenfrost temperature point could be detected based on heat transfer characteristics, rather than collision dynamics.

Laser fusion cutting: The missing link between gas dynamics and cut edge topography

In laser cutting, the fundamental role of the gas flow for melt removal and kerf formation is generally accepted. Beyond this vague understanding, however, the underlying physical mechanisms are not yet fully understood. In particular, detailed data concerning the momentum and heat transfer between the gas and melt have seldom been reported. This study addresses the local interactions between the cutting gas and kerf surface (melt film surface) in a fundamental way based on a combined experimental, theoretical, and numerical approach. Typical solid-state laser cut edges are analyzed considering the characteristic surface structures and the basic influences of the gas flow on the global and local melt movement. Here, apparent structures in the micrometer range indicate the effect of vortical gas structures close to the wall. Theoretical investigation of the gas boundary layer is conducted by semiempirical equations and the transfer of basic results from the boundary layer theory. It is shown that the boundary layer is in transition between the laminar and turbulent flow, and local flow separations and shock-boundary layer interactions primarily induce spatially periodic and quasistationary instability modes. An improved numerical model of the cutting gas flow confirms the theoretical results and exhibits good agreement with experimental cut edges, reproducing relevant instability modes and quantifying the local momentum and heat transfer distributions between the gas and melt. With the knowledge gained about the underlying physical mechanisms, promising approaches for improvements of the fusion cutting performance are proposed.

Internal Heat Transfer Measurement on Metal-Based Additively Manufactured Channels Using a Transient Technique

Abstract The advancements in additive manufacturing (AM) of metals open new possibilities in the design of gas turbine parts. Especially the cooling efficiency of internal channels can be improved with more complex geometries. Naturally, AM channels have a higher surface roughness than conventionally manufactured parts, which influences the cooling air pressure loss as well as the heat transfer. Implementing novel cooling designs using AM can be possible only if the effect of increased surface roughness on the flow and on the heat transfer can be predicted with an appropriate accuracy. The objective of the current study was to measure these parameters experimentally in simple AM channels to build a database for designing complex and efficient cooling designs using the AM technique. A test rig and postprocessing method was elaborated to derive the local internal heat transfer distribution of metal-based AM channels. Six circular single channel coupons made by selective laser melting (SLM) were tested for Reynolds numbers ranging from 20,000 to 50,000. The coupon with the lowest relative roughness shows good agreement with the Dittus–Boelter correlation. All the other coupons show a consistent increase of internal heat transfer and flow friction with the increase of the internal surface roughness.

Assessment of micro-scale heat exchangers efficiency using lattice Boltzmann method and design of experiments

The study is focused on the use of nanofluids in a micro-open tall cavity, which is a type of micro heat exchanger (MHE). The cavity is heated from the bottom sidewall in a sinusoidal pattern, and the effects of four input parameters (Ra, Ha, Kn, and Vf) on heat transfer and irreversibility are investigated using numerical simulations based on Lattice Boltzmann Method (LBM). The findings of the study suggest that the local heat transfer on the bottom sidewall is strongly influenced by Ra and Ha, while the surface distribution of entropy generation is mainly dependent on Kn. The study also shows that the optimization of the magnitude and wavelength of the sinusoidal temperature can improve both local heat transfer and surface distribution of entropy generation. The results of the study provide valuable insights into the design of micro heat exchangers and suggest that the optimization of micro-porous geometries using DOE could lead to increased energy efficiency. The study contributes to our understanding of the complex interactions between input parameters in micro heat exchangers and highlights the importance of considering multiple parameters in the design process.

Experimental and analytical study on local heat transfer distribution between smooth flat plate and free surface impinging jet from a circular straight pipe nozzle

The present study describes the heat transfer characteristics of free surface single-phase circular liquid jet impinging on a thin smooth flat surface subjected to uniform heat flux. The experimental setup is developed to measure the local temperature of a thin stainless steel metal foil using the infrared thermography technique. The effects of nozzle diameter (d = 2.5 to 10.8 mm), nozzle to plate spacing (z/d = 2 to 10), and Reynolds number (Re = 5000 to 24,000) on the local Nusselt number (Nu) distributions are studied. The local Nu is maximum in the stagnation region, and it decreases in the downstream flow following the stagnation region. The nozzle with a larger diameter shows maximum Nu compared to other nozzles. For a given z/d of 2 and a Reynolds number of 15,000, a maximum increment of 67% in local Nu is observed for a 10.8 mm nozzle in comparison to a 2.5 mm nozzle. The local Nu in the viscous boundary layer is found to be a function of the Reynolds number, Prandtl number, dimensionless r/d, and Weber number. The Nu increases with an increase in Reynolds number, however, the effect of the nozzle to plate spacing is negligible. The off-stagnation peak is observed for the 10.8 mm nozzle in the region of 0≤r/d≤0.7. The existing correlations reported in the literature are not able to capture the measured local Nusselt number distribution. Therefore, in the present study, an effort has been made to propose semi-empirical correlations for local Nu in the stagnation and viscous boundary layer region which will incorporate the diameter effect of the nozzle. The proposed correlations are able to predict the local Nusselt number distribution within the maximum deviation of 20%. The comparison between the free surface and submerged jet has been made which suggests that the local Nu for the submerged jet is higher compared to the free surface jet.

Effect of helical coil orientation on flow boiling process

The flow boiling process in helical coils is investigated experimentally to analyze the effects of the orientation of the helical coil on the local heat transfer distribution, pressure drop and critical heat flux. The literature review suggests that little information is available on the influence of the orientation on the heat transfer distribution, pressure drop and critical flux during flow boiling in helical coils. Experiments are performed on horizontal and vertical helical coils with R123 as the working medium. SS-304 tubes of 5.5 mm and 7.5 mm diameter with a coil diameter of 150 mm are chosen in this study. Non-intrusive thermal imaging technique is used to measure the wall temperature. The differential pressure transmitter is used to measure the pressure drop across the test section. The two-phase pressure drop is not affected significantly by the orientation of the helical coils. The wall temperature, heat transfer distribution and the critical heat flux are affected by the orientation of the helical coil. The local and average heat transfer coefficient is higher in vertical oriented helical coils than the horizontal oriented helical coils. The value of heat flux at the burnout is higher in vertical orientation compared to horizontal orientation in both the coils.

Flue Gas Condensation in a Model of the Heat Exchanger: The Effect of the Cooling Water Flow Rate and Its Temperature on Local Heat Transfer

In boiler houses, the biggest heat energy losses are caused by flue gas being released into the atmosphere. Installation of condensing heat exchangers allows reducing the temperature of the flue gas being released, condensation of water vapor, and, thus, efficient use of the waste heat. There are many investigations of average heat transfer in different types of condensing heat exchangers. They indicate also that the cooling water flow rate and its temperature are important parameters defining water vapor condensation efficiency. Investigations of local condensation heat transfer in condensing heat exchangers are very limited. Only recently experimental investigations of the flue gas temperature and Re number effect on local condensation heat transfer in the model of the condensing heat exchanger at a constant cooling water flow rate and its temperature have started being published. In this paper, for the first time, detailed experimental investigations of the cooling water flow rate and its temperature effect on local condensation heat transfer of the water vapor from the flue gas in the model of the condensing heat exchanger (long vertical tube) are presented. The results revealed that at higher flue gas Rein, the effect of the cooling water flow rate and its temperature has a stronger impact on local heat transfer distribution along the test section.

Assessment of Convective Heat Transfer Characteristics for Elliptical-Shaped Pin-Roughened Surface for the Jet Impingement Cooling

Abstract In a jet impingement cooling (JIC) system, the layout of the target surface and length of the jet holes can change both the flow field and the heat transfer characteristics. Elliptical-shaped pins (ESPs) with different heights and layouts on the target surface of the extended jet hole configurations were examined numerically in a jet impingement system. The ESPs were arranged in a staggered and circular form. Normalized nozzle length (Gj/Dj = 1.0, 2.0, 6.0) and normalized pin height (Hp/Dj = 0, 0.167, 0.417, 0.667) were investigated as geometric parameters. Also, the effect of different pin layouts (R1, R2, R3) on heat transfer dissipation was studied by changing the number of pin rows in particular configurations. A numerical model was developed and verified with experimental and numerical data from the literature. Numerical analyses were conducted with the shear stress transport (SST) k–ω turbulence model taking the boundary conditions into account under turbulent flow conditions (16,250 ≤ Re ≤ 32,500). Nusselt (Nu) numbers, pressure drop, and the thermo-hydraulic performance of the physical model were quantitatively researched to elucidate the underlying mechanisms of enhanced heat transfer by the ESPs. Results were compared with the orifice surface (Hp/Dj = 0 and Gj/Dj = 6.0). Results showed that area-averaged Nu number on the target wall increased up to 35.82% for Re = 16,250 by R2_Gj/Dj = 1.0 and Hp/Dj = 0.167 compared to the conventional JIC system. The performance evaluation criterion (PEC) was used to analyze the thermo-hydraulic performance of the examined physical models. According to the PEC values, the most feasible parameters for all Re numbers were R3_Gj/Dj = 1.0 and Hp/Dj = 0.167. Furthermore, increasing the number of pin rows in the channel also increased the uniformity of the local heat transfer distribution according to Nu contours.

Simultaneous high-accuracy measurements of local heat transfer and phase distribution during convective condensation using non intrusive diagnostic tools

An experimental setup was built to measure simultaneously the profiles of the phase distribution and the heat transfer coefficient in a minitube during convective condensation of HFE7000 at low mass velocities. The aim is to overcome the lack of measurements available in the literature in these conditions, mainly due to the difficulty to measure accurately heat transfer and phase distribution simultaneously. The metrology combines two high-precision measurement techniques allowing accurate measurement of the condenser wall temperature by infrared method and the thickness of the liquid film flowing inside the condenser by interferometric or confocal method. A specific protocol was developed to reduce the uncertainty on the wall temperature down to 0.08 °C. The flow regime varies from smooth annular to wavy annular with mean thickness of the liquid film less than 100 μm when the mass velocity is increased from 5 to 30 kg m−2 s−1. The heat transfer coefficient increases locally by up to 30% as the mass velocity increases. These profiles were than compared to the ones of both Nusselt theory and 5 correlations selected from the literature. None of them allow an accurate prediction of the heat transfer coefficient obtained experimentally.

Heat transfer distribution and flow characteristics in a channel with perforated-baffles

The current paper reports the prediction of heat transfer performance of a channel with a modified perforated-baffle in a square wings (SW-PB) form. Numerical results include flow/temperature field, local heat transfer distribution, and thermal performance of a channel installed with the transverse solid baffle (TB), perforated-baffle (PB), and perforated-baffle with square wings (SW-PB). The simulation results demonstrated that TB brought a large recirculation flow, PB induced small recirculation and SW-PB produced several impinging jets as well as recirculation flows. Heat transfer rates given by PB were lower than those provided by TB and SW-PB by around 6.8% and 7.3%, respectively which were accompanied by lower friction losses by about 11.8% and 3.6%, respectively. Although, SW-PB gave higher heat transfer, the assessment of thermal performance factor (TPF) showed that the benefit from lower friction losses was more important. Under the same pumping power, TPFs assisted by SW-PB were higher than that assisted by PB and TB by 6.0% and 3.3%, respectively. The highest TPF of 1.2 was captured by SW-PB at Re = 9,000.

Heat Transfer of a Metal Foamed Flat Plate Impinged by Multiple Jets

The present study focuses on the measurement of the local heat transfer distribution of a smooth flat plate, a flat plate with attached metal foam, and detached metal foam under an inline array of multiple jets using thermal imaging and the thin metal foil technique. After impingement, jet flow exits from all directions. The range of the Reynolds numbers covered in the study is 2500 to 15,000. The impinging distance of to is studied. A 4-mm-thick aluminum metal foam having 92% porosity and 20-pore-per-inch pore density is used. The contribution of the convection and conduction heat for a flat plate with metal foam is studied using a flat plate with a detached foam configuration. The effect of the impinging distance and Reynolds number on the local, spanwise-averaged, and overall averaged Nusselt numbers is studied. The presence of the 4 mm metal foam enhances the local heat transfer in comparison with a smooth flat plate due to conduction heat transfer without the addition of extra hydraulic resistance to the jet flow. The enhancement of the heat transfer due to the presence of the metal foam is quantified by the enhancement factor , and it is found to be around 1.6.

Influence of metal foam thickness on the conduction and convective heat transfer for a flat plate with metal foam impinged by a rectangular slot jet

The local heat transfer distribution is measured for the case of a flat plate with a porous metal foam impinged by a rectangular slot air jet. The main objective is to separate the convection and conduction heat transfer effects in a flat plate with metal foam. For this purpose, flat plate with attached and detached metal foam are studied. The effect of the thickness of the metal foam on the convection and conduction heat transfer is also investigated. An open-cell aluminium metal foam having a porosity of 92% and pore density of 20 PPI (pores per inch) is studied. The thickness of the metal foam studied is 4 mm, 8 mm and 12 mm. Reynolds number ranges from 5200 to 12,000 and impinging distance varies from 2 to 10 times the width of the rectangular slot. The local Nusselt number of a flat plate with attached and detached metal foam is independent of the impinging distance irrespective of the metal foam thickness. The impinging jet on the flat plate with metal foam experiences extra hydraulic resistance depending on the thickness of the metal foam. The 12 mm thick metal foam offers the highest extra hydraulic resistance. However, the 4 mm thick metal foam offers negligible extra hydraulic resistance. The flat plate with detached metal foam experiences attenuation in the convective heat transfer due to extra hydraulic resistance. However, the flat plate with attached metal foam experiences conduction heat transfer due to metallic foam along with attenuation in the convective heat transfer due to extra hydraulic resistance. The attenuation in the convection heat transfer and conduction heat transfer due to the presence of the metal foam is quantified by the attenuation factor and enhancement factor respectively. The average attenuation factor is 1.12, 0.96 and 0.84 for 4 mm, 8 mm and 12 mm respectively. The enhancement factor for the metal foam having 4 mm, 8 mm and 12 mm are 1.91, 2.20 and 2.90 respectively. Hence, the effect of the enhancement due to conduction heat transfer dominates over the effect of the attenuation in the convective heat transfer due to extra hydraulic resistance. Under slot jet impingement, a flat plate with 4 mm, 8 mm and 12 mm thick metal foam shows 2.02, 2.12 and 2.41 times overall enhancement in comparison with the smooth flat plate respectively. Region-wise correlations for Nusselt number are developed using multiple regression analysis.

Heat Transfer from a Hybrid Pulsating and Swirling Air Jet Impingement

With decrease in size of electronic equipment’s, effective heat removal is a major challenge these days. Present work focuses on implementation of a novel hybrid method which helps attain heat transfer enhancement. An experimental investigation is carried out to study effect of pulse air jet on local heat transfer distribution of a flat surface. Effect of nozzle to plate distance (z/d = 2 to 6), Reynolds number (5000 to 9000), pulsating frequency (f = 0.07 to 2.03 hz) and Strouhal number (Sr = 6.7e-5 to 0.0029) are studied with constant nozzle diameter. Thin foil technique is used to estimate local heat transfer characteristics using IR thermal infrared imaging technique. Nusselt number distribution is plotted for all the cases using information got from IR image. It is observed that at lower frequency rate and low Strouhal number heat, transfer rate is more effective. A novel hybrid method to improve heat transfer rate is introduced in this study using Pulse combined with swirl technique. This method involves introducing swirler of specified twist ratio into nozzle subjected to air pulse. Experiments show that this novel hybrid method improves heat transfer rate at all the above-mentioned conditions.

Experimental assessment of the gap width effect on turbulent flow and forced convective heat transfer around a single rod suspended in a channel

The present work presents the experimental campaign carried out to assess the convective heat transfer around a single rod suspended in a rectangular channel under axial turbulent flow. The distance between the rod and the upper wall of the channel forms a rod-wall gap, whose width, d, can be varied. The experiments were performed for the dimensionless gap widths that yield W/D = 1.050, 1.100, 1.150 and 1.200. Hot-wire anemometry was used to measure mean velocities, the RMS velocity fluctuations and the velocity spectra. The rod is equipped with a heated cell that provides the local temperature and convective heat transfer coefficient distributions around the rod’s surface. The local convective heat transfer distribution is in good agreement with the literature. The lowest value for the heat transfer coefficient was not found right in the narrow gap for W/D = 1.050, but, slightly away from the gap. As W/D increases, the minimum local convective heat transfer coefficient occurred closer to the narrowest gap width. The Reynolds-Colburn analogy was investigated for all configurations through the measured convective heat transfer coefficient and the skin friction distributions. A linear relationship was found between the Stanton number and the skin friction coefficient. Moreover, data scattering was seen to decrease significantly when the local velocity was used instead of the global one. It suggests that the heat transfer process is governed, mainly, by the local parameters.

Experimental study of turbulent flow heat transfer and pressure loss over surfaces with dense micro-depth dimples under viscous sublayer

Presented are experimentally-measured heat transfer and pressure loss characteristics for dimpled surfaces, placed along one surface of a channel, with different ratios of dimple depth to channel height δ/H, and for Reynolds numbers ranging from 10,000 to 70,000. With the same relative dimple spacing to dimple print diameter, and the same ratio of dimple depth to dimple print diameter of δ/d = 0.20, experimental results for arrays of spherical indentation dimples with micro depths of δ = 0.6 and 1.0 mm (δ/H = 0.03 and 0.05) are compared with thermal characteristics for arrays of larger dimples with δ = 4.0 mm and δ/H = 0.20. Local surface heat transfer distributions are measured using transient liquid crystal thermography. Additional steady-state experiments are also employed to provide overall heat transfer enhancement and friction factor augmentation values. Results indicate that Nusselt number ratios and friction factor ratios of a turbulent flow in the channel are dependent upon the ratio of dimple micro depth to channel height, which are different findings from previous studies with larger-depth dimples in the cooling channels. As such, the present data for micro-depth dimples with ratios of dimple depth to channel height δ/H of 0.03 and 0.05 are firstly provided. The importance of a critical Reynolds number is demonstrated by the results, which is determined when the ratio of viscous sublayer layer thickness to the dimple depth is equal to unity, and has been ignored in the previous dimpled channel heat transfer design. Nusselt number enhancements for the micro dimples are generally lower at lower Reynolds numbers, but increase as the Reynolds number becomes larger and approaches the critical Reynolds number. Nusselt number enhancements are then approximately invariant with Reynolds number, when Reynolds number then exceeds the critical value.

Development and Assessment of an Isotropic Four-Equation Model for Heat Transfer of Low Prandtl Number Fluids

In the simple gradient diffusion hypothesis, the turbulent Prandtl number (Prt) with a constant of 0.85 is difficult to accurately predict for liquid metals having low Prandtl numbers (Pr), while a four-equation model can improve this solution by introducing the turbulence time-scale into the calculation of turbulent thermal diffusivity. However, the four-equation model’s transport form and numerical stability are so complex that suitable commercial code is lacking. Therefore, an isotropic four-equation model with simple Dirichlet wall boundary conditions is built in the present work. Based on the open-source computational fluid dynamics program OpenFOAM, the fully developed velocity, temperature, Reynolds stress, and heat flux of low Pr fluids (Pr = 0.01–0.05) in the parallel plane are obtained by numerical simulation. The results show that the time-average statistics predicted using the present four-equation model are in good agreement with the direct numerical simulation data. Then, the isotropic four-equation model is used to analyze the flow and heat of liquid metal (Pr = 0.01) in a quadrilateral infinite rod bundle. The numerical results are compared with the various and available experimental relationships. The Nusselt numbers calculated using the isotropic four-equation model are betweenness the available correlations, while the turbulent Prandtl number model using a constant of 0.85 over predicts heat transfer. More detailed local heat transfer phenomena and distribution of low Pr fluids are obtained using the present isotropic four-equation model.

Simultaneous High-Accuracy Measurements of Local Heat Transfer and Phase Distribution During Convective Condensation Using Non Intrusive Diagnostic Tools

Simultaneous High-Accuracy Measurements of Local Heat Transfer and Phase Distribution During Convective Condensation Using Non Intrusive Diagnostic Tools

Influence of synthetic air jet temperature on local heat transfer characteristics of synthetic air jet impingement

The influence of synthetic air jet temperature on local heat transfer distribution of a synthetic air jet impinging on a smooth flat surface is investigated experimentally. The actuator frequency (f = 100 Hz to 300 Hz) and jet to orifice spacing (z/d = 1 to 8) is considered for the present study. The local heat transfer study is obtained using thin foil and IR thermal imaging technique. At z/d = 1, the Nusselt number characteristics are found approximately 2 times greater when synthetic air jet temperature is considered as jet air temperature instead of surrounding air temperature for all the frequencies. The experimental results observed that heat transfer distribution is strongly influenced by synthetic air jet temperature especially at lower orifice to jet distance.

Systematic Comparison on Convective Heat Transfer Characteristics of Several Pin Fins for Turbine Cooling

This paper is focused on the heat transfer augment ability and friction factor of different cross-section pin fins. An experimental study is conducted in a wide rectangular channel. The steady-state thermochromic liquid crystals (TLC) method is applied to measure the tested surface temperature. Nine sets of pin fins are employed in the experiment. The nominal diameter of all pin fins is the same value. Nine sets of pin fins have three roundness shapes (Circle, Ellipse and Oblong), three streamline shapes (Dropform, NACA and Lancet) and three quadrangle shapes (Diamond, Diamond-s and Square), respectively. The arrangement parameters of all nine shapes are kept the same. As they have the same nominal diameter and arrangement, the channel blockage ratio is the same for each pin fin set. Reynolds numbers range from 10,000 to 60,000. The pressure losses of pin fin arrays are measured to obtain friction factor. Meanwhile, the overall thermal performances of all nine sets are also considered and compared. The results show heat transfer enhancement abilities of quadrangle shape pin fins are relatively higher than the roundness and streamline shapes. Diamond-s pin fins present the largest averaged Nusselt number and overall thermal performance on the endwall for all the nine pin fins under different Re. Concerning overall thermal performance, the traditional Circle pin fin is the second best. The pressure loss of streamline shape pin fins is the lowest in these three shape types. Moreover, the characteristic of local heat transfer distribution varies substantially for different pin shapes at low Re.