Highest Average Heat Transfer Coefficient Research Articles

Experimental study of the compact brazed heat exchanger on the thermal-hydraulic performance

ABSTRACT In this article, the heat transfer and pressure drop characteristics of a compact brazed heat exchanger (CBHE) are experimentally investigated. With the data obtained from the experiments, analyses were carried out to evaluate heat transfer, effectiveness, and pressure drop performance. From the results, it was seen that as the mass fluid rate increases, the heat transfer rate also increases. The highest heat transfer rate is 18,087 W for a mass flow rate of 210 g/s. This value is for Reynolds number 2125.72. The highest average heat transfer coefficient value in the experiments is 2106 W/m2K. In the experiments, it was seen that the efficiency values increased as the Reynolds number increased. The highest effectiveness value is 0.49. As the Reynolds number increased, the pressure drop also increased. The highest pressure loss value was found to be 20,377 Pa. Based on the results obtained from the experiments, linear correlation analysis was also performed to determine whether there is a trend between pressure drop and Re and heat transfer rate and mass flow rate. The correlation coefficient between pressure drop and Re and heat transfer rate and mass flow rate are 0.993217 and 0.979179, respectively.

Improved Heat Transfer Capabilities of Nanofluids—An Assessment Through CFD Analysis

AbstractConventional fluids used in fission‐based water‐cooled nuclear reactors have lower heat transfer coefficients (HTCs) and thermal conductivity, which has led researchers to explore high‐performance fluids that can enhance heat transfer in routine operation and prevent core meltdown in the case of accidents. It is important to investigate a wide range of fluids that can help designers improve thermal hydraulic characteristics, such as HTC, critical heat flux, and minimum departure from nucleate boiling ratio (MDNBR). In this study, the effectiveness of nanofluids in enhancing heat transfer parameters, including thermal conductivity and heat capacity, was investigated. Four different nanofluids (Al2O3–H2O, ZrO2–H2O, Ag–H2O, and Si–H2O) with pure water as the primary coolant in an HPR‐1000 nuclear reactor were compared using computational methods. Due to computational limitations, only the flow channel among four fuel rods with the highest power density in the core was simulated using Eulerian computational fluid dynamics. The results of this study show that silver water (Ag–H2O) nanofluid outperformed other nanofluids and pure water. It had a higher average HTC and MDNBR, with a 67.15 % and 45.23 % improvement, respectively, compared to pure water. The fuel rod wall temperature was also reduced by 28.5 K with Ag–H2O compared to water. Comparison of current simulated results with literature data shows a good agreement.

A comparative analysis of ammonia and supercritical carbon dioxide in horizontal microchannels

Supercritical carbon dioxide (sCO2) and ammonia (NH3) can be used as coolants in high power-density microelectronics. sCO2 at the micro scale provides enhanced heat transfer solutions for a range of microelectronics cooling applications·NH3 can also potentially provide appropriate cooling solutions as it has favorable thermophysical properties at high pressure, and it can be used as an energy carrier as it contains Hydrogen atoms. Near its critical and pseudocritical condition sCO2 exhibits abrupt changes in its thermophysical properties and at similar pressures liquid NH3 too has adequate properties that make it a good coolant. While sCO2 is non-toxic, NH3 is a toxic substance and should only be used in closed loop micro heat exchangers.The current study compares the convective heat transfer performance of supercritical carbon dioxide (sCO2) and ammonia for microchannel cooling. A total of 144 numerical cases inside a square microchannel of hydraulic diameter of 200 µm with a total length of 52 mm and a heated length of 50 mm at constant surface temperatures in laminar flow conditions were examined. Two scenarios were investigated, in one the inlet mass flux was kept constant at 100, 150, and 200 kg/m2s, and in the second the pressure drop across the microchannel was maintained at 200, 400, and 600 Pa. Both scenarios were investigated at pressures of 8 and 10 MPa and at surface temperatures of 32, 40, 50, 60, 70, and 80 °C. In all cases the inlet temperature of both fluids was kept at 21 °C. At the same mass flux, NH3 yielded a higher average heat transfer coefficient (havg) but at the same pressure drop, sCO2 resulted in higher havg at certain surface temperatures. The havg for NH3 didn’t show significant variation but for sCO2 it showed significant change with surface temperature, and this was due to the significant change in the thermophysical properties of sCO2. The havg showed a mixed behavior with respect to the pumping power. The coefficient of performance (COP) was as high as 600,000 for sCO2 and it was significantly higher than that for NH3. Near the pseudocritical region, the COP for sCO2 witnessed a significant improvement, more than double, for 8 MPa compared to 10 MPa.

Experimental analysis of a compact cooling system containing an enhanced-surface spray heat sink

This paper examines the performance of a heat sink that combines an evaporator and an expansion device into a single component within a compact vapor compression refrigeration system designed for applications with high heat fluxes. The liquid refrigerant at high pressure expands through orifices, generating a spray that impacts a copper heater. The heated surface is enhanced with laser-micromachined micropillars featuring a square cross-section to improve the thermal conductance of the boiling liquid film. Qualitative evaluations of the enhanced surfaces using interferometry analyses determined the heat transfer surface area and assessed the effectiveness of the fabrication technique. The experiments involved using R-134a, a two-orifice expansion device, and enhanced surfaces with pillar edge sizes ranging from 200 to 600μm. This analysis encompasses the cooling system’s thermodynamic performance, including compressor power and coefficient of performance, as well as steady-state heat transfer parameters such as convection coefficient and critical heat flux. The system achieved a cooling capacity of 230 W (35.9 W/cm2), with the heater surface temperature staying below 22.9 °C and the highest average heat transfer coefficient reaching as high as 55 kW/(m2-K). This represents a 129% increase in thermal conductance compared to mirror polished surfaces, leading to a significant reduction in the temperature of the heated surface. Additionally, the area-corrected heat transfer coefficient is computed to verify the thermal resistance reduction unrelated to the surface area enhancement, and a 60% reduction is found. Furthermore, although this heat transfer enhancement shows minor to no influence on the thermodynamic performance of the refrigeration system, the heater cooling efficiency, defined as the ratio of the enthalpy rise delivered by the heat source to its ideal maximum, increased by 13%. The findings are supported by high-speed video sequences illustrating the boiling heat transfer features.

Study of a non-reacting hypersonic flow over an external cavity with flow injection using DSMC method

In the current study, a DSMC solver is utilized to study a rarefied external cavity flow. The main focus of this research is mainly on the properties of flow near the rear wall of the cavity. It is observed that the flow temperature near the wall increases by increasing the free stream Mach number. Also, the cavity length-to-depth (L/D) ratio affects the flow structure dramatically in a constant Mach number. Injection from the rear wall of the cavity is studied. Results show that injection does not affect cavity vortex location. Local density and x-velocity increase is observed at the point of injection. Nonmonotonic y-velocity behavior is seen as a result of cavity vortex interaction with low-momentum injection fluid. Dimensionless temperature is decreased significantly due to injection. So, it can be treated as a remedy to decrease surface heat transfer coefficient. No significant impact is seen on the wall pressure coefficient and friction coefficient due to injection. The average number flux is increased due to injection. Higher L/D ratios result in a higher average heat transfer coefficient while having lower gradients along the wall. Enhancing the injection velocity would decrease the heat transfer coefficient and promotes cooling effects. Cold-to-hot heat transfer occurs inside the cavity and heat flux lines show that injection, decreases the flow temperature inside the cavity near the rear wall in comparison with no injection case.

Local heat transfer characteristics of circular multiple air jet impinging on a semicircular concave surface

ABSTRACT The local heat transfer characteristics of circular multiple air jet impingement on semicircular concave surface were studied experimentally. For constant Reynolds number (Re = 9,000), the effect of jet to concave surface spacing (z/d = 1, 4.2, 6), ratio of curvature (D/d = 4.28, 6, 8.6), jet-to-jet spacing (s/d = 2.4, 4, 5.6), and number of jets were considered. The local heat transfer characteristics were determined by using infrared thermal camera and thin foil method. Lower curvature value ratio (D/d = 4.28) resulted higher average heat transfer coefficient due to broader spread of heat transfer distribution, whereas higher curvature ratio (D/d = 8.6) resulted in lower average heat transfer coefficient due to steeper heat transfer distribution in the stagnation area. At lower D/d, z/d, and s/d with seven jet configuration, the heat transfer coefficients were found effective as compared to other configurations due to broad heat transfer distribution and less interaction between the jets. At given D/d and s/d configuration, the stagnation Nu at θ = 0° do not coincide with jet hole center axis and this shift is more pronounced at higher z/d value. This could be due to additional push before jet impinging on concave surface. The secondary peaks were observed between two neighboring stagnation regions in Nu distribution along the center longitudinal line for lower z/d and higher s/d values. This resulted due to upwash that was obtained from jet interactions related to collisions with the wall jets. The correlations of stagnation line average Nu and overall average Nu were obtained. The coefficient of variance of local heat transfer coefficient for each configuration was determined, and correlation was proposed to analyze the degree of non-uniformity.

CFD Simulation of Heat Transfer Characteristics in a Rectangular Duct with Trapezoidal Turbulence Promoters

The heat transfer rate from the hot surface to the coolant flow increases by placing the ribs on the surface of the coolant flow passage. The effect of turbulence promoters with different angle of attack and with a fixed tapered angle on the heat transfer characteristics in a rectangular stationary duct is considered for this study. Here turbulence promoters increase the heat transfer in the duct. The examination has been conducted on the rectangular smooth duct and rectangular duct with inverted V-shaped ribs of a different angle of attack. The rib angle attack of 45º, 60º, 90º and tapered angle 20º (fixed) on the top surface of the rib are considered. The rib blockage ratio and pitch ratio are 0.0553 and 10 respectively. The heat transfer characteristics are compared with Reynolds number of 6000. The heat transfer and computational flow analysis are conducted by k-epsilon turbulence model in ANSYS FLUENT. The tapered angled cross-sectional inverted V-shaped ribs of 60º shown high average heat transfer coefficients among the 45º, 60º and 90º smooth duct.

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).

Flow and Heat Transfer Simulation Analysis of 3D Compact Heat Exchanger

Compact heat exchangers (CHEs) are one of the most commonly used heat exchangers in the industry due to their superior advantages over other types of heat exchangers. Various geometric (fin spacing, tube inclination angle, etc) and process (such as flow velocity, temperature, etc) parameters affect the performance of such compact HEs. This research aims to examine the effects of fin spacing, tube inclination angle, and airflow velocity on heat transfer and pressure drop performance of CHE in both inline and staggered configurations. A three-dimensional (3D) numerical method with the aid of Ansys FLUENT software was carried out for the laminar flow condition. Based on the obtained results, the highest average heat transfer coefficient was observed at 120° for both tube arrangements while the lowest average pressure drop penalty is at 30°. Therefore, the recommended inclination angle when high heat transfer is needed is at 120° while if the pumping power is the major problem, 30 °or 150° is recommended. based on the London area goodness factor (j/f), 30° and 150° show the highest value for both configurations. The j/f factor decreases with the increase of Reynolds number for both configurations. In addition, 120° shows the lowest j/f which can be due to the high pressure drop.

Heat transfer enhancement during condensation of water steam on inclined pipe

This paper presents experimental study of heat transfer during film condensation of saturated water steam on the outer surface of the inclined pipe by gradient heatmetry. Heat flux per unit area was measured by gradient heat flux sensors made of a single-crystal bismuth. The experimental results are presented in the graphs of heat flux per unit area dependence on time and azimuthal angle. The highest average heat transfer coefficient during condensation of α = 6.94 kW/(m2 • K) was observed when the pipe was inclined at the angle of ψ = 20 °. This value exceeds one obtained on a vertical pipe by 14.9 %. Heat transfer enhancement during condensation of saturated water steam on inclined pipe is associated with changes in condensate film flow. Another part of experiments was made by simultaneously using of gradient heatmetry and condensate flow visualization. Experimental results confirmed the applicability and high informative content of proposed comprehensive method. Comprehensive study of heat transfer during condensation confirmed that heat flux per unit area pulsations may be explained by the formation of individual drops, their coalescence, and drainage from the sensor surface.

A theoretical study of condensation heat transfer in tubes with novel cross-sections

This paper presents a numerical study of stratified flow condensation heat transfer in tubes with novel cross-section designs. A design framework of the novel cross-section geometry was developed by first studying and analyzing the condensate film distribution in a circular tube. It was found that the circular arc diameter and orientation have effects on the condensate film thickness. Based on this understanding, four different cross-section designs (models N1, N2, N3 and N4) formed by connecting circular arcs of different curvatures at various orientations were proposed. The heat transfer performances of these models were compared to a circular tube of the same perimeter. Our simulations show that all models possess higher average heat transfer coefficients than a circular tube when no accumulated condensate layer is present. Among all the models proposed, model N4 exhibits the largest heat transfer enhancement ratio of 1.42 at ∊ = 0.96. The design of model N4 utilizes small circular arcs which are arranged in an orientation that increases the effect of the gravitational force, providing significant condensate film thickness reduction over a large area. This study not only demonstrates the possibility of enhancing condensation heat transfer by simply varying the tube curvature but also provided a design guideline which can be employed for the development of novel cross-section tubes that can be fabricated by advanced manufacturing techniques such as selective laser melting.

Computational Investigation on Natural Convection Heat Transfer in Vertical Plate Fin Arrays

Objectives: To perform computational studies on different combinations of plate fin arrays of different shapes for studying their performance in natural convection heat transfer. To compare the heat transfer performance of uniform notched fins with hybrid notched fins and also performance of normal notched fins with inverted notched fins. Methods: SolidWorks Flow Simulation software is used for the present computational studies. Three types of vertical plate fin arrays viz. plain rectangular fin, rectangular fin with square notch and rectangular fin with semi-circular notch are used with their seven combinations. These fin array combinations are: plain rectangular, rectangular square notched, rectangular semi-circular notched, rectangular inverted square notched, rectangular inverted semi-circular notched, hybrid rectangular square-semi-circular notched and hybrid rectangular inverted square-semi-circular notched. Findings: The highest average heat transfer coefficient of 7.82 W/m2K is found to be for inverted hybrid square-semicircular notched fin array. This value of heat transfer coefficient is about 8% higher than compared to that with plain rectangular plate fin array (7.25 W/m2K) subjected to same operating conditions. Average heat transfer coefficients for inverted plate fin array combinations are high as compared with the non-inverted combinations. Also, hybrid plate fin arrays, whether inverted or not, show higher heat transfer coefficient values as compared with uniform notched fin arrays. Novelty : Seven types of plate fin array combinations are studied computationally. SolidWorks Flow Simulation software is used very effectively for this 3D CFD analysis. The behaviour of fluid i.e. air during natural convection flow is studied using air flow velocity trajectories. Surface contours are effectively used for studying variation in surface temperature and heat transfer coefficient at all locations of fin arrays. Keywords Natural convection, plate fin array, heat transfer coefficient, CFD, heat transfer enhancement

Transient response of a plate-fin-and-tube heat exchanger considering different heat transfer coefficients in individual tube rows

Experimental studies of multi-row plate-fin heat exchangers show that the highest average heat transfer coefficient on the air side occurs in the first row of tubes. In the next rows of tubes up to about the sixth row the heat transfer coefficient decreases.It is necessary to find the relationships for the air-side Nusselt number on each tube row to design a plate-fin and tube heat exchanger (PFTHE) with the optimum number of tube rows. The air-side Nusselt number correlations can be determined experimentally or by CFD modeling (Computational Fluid Dynamics). The paper presents a new method of modeling the transient operation of PFTHE, considering that the Nusselt numbers on the air side of individual tube rows are calculated from different empirical relationships. CFD modeling of the PFTHE was used to establish the heat transfer coefficients on individual tube rows. A transient model of a double-row, two-pass PFTHE was developed, considering different heat transfer coefficients in the first and second row of tubes. The influence of the adoption of different coefficients on the heat flow rate exchanged between water and air in the first and second row of tubes in two passes of the heat exchanger was shown.

Effect of fin pitch on the filmwise condensation of steam on three-dimensional conical pin fin arrays: A comparative study

An experimental investigation on the use of three-dimensional conical pin fins to enhance filmwise condensation of steam on vertical plates was carried out. Nine surfaces with conical pin fin arrays were fabricated by Selective Laser Melting and tested in a condensation chamber. The conical pin fin arrays have the same fin base diameter but are of different fin heights and fin pitches. The effects of fin pitch on the condensation heat transfer performance of the conical pin fin arrays were examined. A detailed comparative study was also carried out to evaluate the thermal performances of the conical, sinusoidal and cylindrical pin fin arrays fabricated by Selective Laser Melting. Based on our investigation, it was determined that the condensation heat flux and heat transfer coefficient of the conical pin fin specimens increase as the fin pitch increases from 1.25 mm to 1.67 mm. However, with a further increase in the fin pitch from 1.67 mm to 2.50 mm, reductions in the heat flux and heat transfer coefficient were recorded. The highest thermal enhancement factor of 2.46 was achieved with the conical pin fin specimen of 1.28 mm fin height and 1.67 mm fin pitch. Visualization studies of the static condensate retention height show that the change in fin pitch has more significant effect on the condensation retention height as compared to the change in fin height. A fin analysis was performed to determine the average heat transfer coefficients of the conical pin fins. In addition, a thermal enhancement-to-area enhancement ratio was also proposed to evaluate the condensation performances of conical pin fin arrays. Our results show that significantly higher average heat transfer coefficients and thermal enhancement-to-area enhancement ratios were obtained by the conical pin fin surfaces as compared to the sinusoidal and cylindrical pin fins. Due to the significant surface tension effect and reduced condensate flooding, the conical pin fin structure is shown to be the best geometry for enhancing natural convection condensation heat transfer as compared to the sinusoidal and cylindrical pin fins.

Numerical investigations on flow and heat transfer of swirl and impingement composite cooling structures of turbine blade leading edge

In this paper, four swirl and impingement composite cooling structures are established to deeply study the flow and heat transfer characteristics, where the swirl nozzles and impingement nozzles are reasonably arranged. Numerical simulation is conducted by solving the Reynolds Averaged Navier-Stokes (RANS) equations with the standard k-ω model. Meanwhile, numerical results are compared with the cooling behaviors of swirl cooling and impingement cooling under the same condition. Results revealed that the pressure distribution of four composite cooling structures is quite different from that of swirl cooling and impingement cooling. Hence, the nozzle mass flow ratio distribution of composite cooling structures displays a large fluctuation with the variation of the nozzle location, which has an influence on the flow and heat transfer characteristics. Moreover, the heat transfer characteristics of swirl and impingement composite cooling combine the advantages of impingement cooling and swirl cooling, where there both exists extremely high local heat transfer regions and uniform heat transfer regions. As for composite cooling 3 and composite cooling 4, the alternate locations of impingement nozzles and swirl nozzles could effectively increase the band-shaped high heat transfer area. Meanwhile, the low heat transfer area caused by the continuous arrangement of impingement nozzles is reduced. Among four composite cooling structures, the composite cooling 4 has the highest average heat transfer coefficient and the minimum pressure loss. The globally average heat transfer of composite cooling 4 is 3.49% lower than swirl cooling but is 19.12% higher than impingement cooling. Its total pressure loss is 4.29% lower than swirl cooling and is slightly lower compared with impingement cooling.

Transient behavior of a plate-fin-and-tube heat exchanger taking into account different heat transfer coefficients on the individual tube rows

Experimental studies of multi-row plate-fin heat exchangers show that the highest average heat transfer coefficient on the air side occurs in the first row of tubes when the air velocity in front ofthe exchanger is less thanapproximately 3.5 m/s. In the subsequent rows of tubes up to about the fourth row the heat transfer coefficient decreases. In the fifth and further rows, it can be assumed that the heat transfer coefficient is equal in each tube row. It is necessary to find the relationships fortheair–side Nusselt number on each tube row to design a plate–fin and tube heat exchanger(PFTHE) with the appropriate number of tube rows. The air–side Nusselt number correlations canbe determined experimentally or by CFD modeling (Computational and Fluid Dynamics). The paper presents a newmathematical model of the transient operation of PFTHE, considering that the Nusselt numbers on the air side of individual tube rows are different. The heat transfer coefficient on an analyzed tube row was determined from the equality condition of mass– average air temperature differences on agiven tube row determined using the analytical formula and CFD modeling. The results of numerical modelingwere compared with the results of the experiments.

Transient behavior of a plate-fin-and-tube heat exchanger taking into account different heat transfer coefficients on the individual tube rows

Plate-fin and tube heat exchangers (PFTHE) are made of round, elliptical, oval or flat tubes to which continuous fins ( lamellas) are attached. Liquid flows inside the tubes and gas flows outside the tubes perpendicularly to their axes and parallel to the surface of continuous fins. Experimental studies of multi-row plate-fin and tube heat exchangers show that the highest average heat transfer coefficient on the air side occurs in the first row of tubes when the air velocity in front of the exchanger is less than approximately 3.5 m/s when a Reynolds number based on an equivalent hydraulic diameter equal to the distance between tube rows in the direction of air flow is less than 10,000. In the subsequent rows of tubes up to about the fourth row the heat transfer coefficient decreases. In the fifth and further rows, it can, that the heat transfer coefficient is equal in each tube row. It is necessary to find the relationships for the air-side Nusselt number on each tube row to design a PFTHE with the appropriate number of tube rows. The air-side Nusselt number correlations can be determined experimentally or by CFD modeling (Computational and Fluid Dynamics). The paper presents a new mathematical model of the transient operation of PFTHE, considering that the Nusselt numbers on the air side of individual tube rows are different. The heat transfer coefficient on an analyzed tube row was determined from the equality condition of mass- average air temperature differences on a given tube row determined using the analytical formula and CFD modeling. The results of numerical modeling were compared with the results of the experiments.

Filmwise condensation of steam on sinusoidal pin fin arrays: Effects of fin height and fin pitch

An experimental investigation on the use of sinusoidal pin fins to enhance filmwise condensation of steam on vertical plates was carried out. Nine surfaces with pin fin arrays were fabricated by Selective Laser Melting (SLM) and tested in a condensation chamber. The pin fin arrays have the same fin base diameter (db) but are of different fin heights (l) and fin pitches (p). At the same fin pitch (p = 1.25 mm and 1.67 mm), the heat flux (q″) and condensation heat transfer coefficient (h) increase as l increases from 1.25 mm to 1.66 mm. However, with further increment in l from 1.66 mm to 2.49 mm, reductions in q″ and h were observed. At the same fin heights of l = 1.25 mm and 1.66 mm, an increase in p from 1.25 mm to 1.67 mm has negligible effects on q″ and h. However, with further increment in p from 1.67 mm to 2.50 mm, significant reductions in q″ and h were observed. Visualization studies of the static condensate retention height (Have) show that two distinct flooding regions can be identified for sinusoidal pin fin surfaces. The static condensate retention height of both regions decreases with increasing p but remains unchanged with varying l. A fin analysis was performed to determine the average heat transfer coefficients (ht) which considers the total heat transfer areas (At) of the enhanced surfaces. Our results show that the highest thermal enhancement factor (η) of 1.86 is achieved with Specimens S4 and S5 whereas Specimens S3 has the highest average heat transfer coefficient (ht). In comparison with cylindrical pin fin surfaces, the sinusoidal pin fin surfaces show better heat transfer performances at the same p/l ratio.

The role of disturbance waves in nucleate boiling in annular flow

In annular two-phase gas-liquid flow, the liquid film on the wall consists of relatively quiescent substrate regions which are traversed by large amplitude, high velocity waves known as disturbance waves. The turbulent disturbance wave regions have relatively high average heat transfer coefficients (low average wall temperatures) compared to the (probably laminar) substrate regions. Nevertheless, there is evidence that nucleate boiling (necessitating a higher wall temperature) occurs first in the wave regions. This paper explores the hypothesis that wall temperature fluctuations due to turbulence in the disturbance waves are of sufficient magnitude to give localized triggering of nucleation sites and hence nucleate boiling. This hypothesis was explored using Computational Fluid Dynamics (CFD). The turbulence was modelled using wall-resolved LES. The results lend weight to the hypothesis that the nucleate boiling observed in disturbance waves is due to transient local high temperatures induced by the turbulence.

Investigation of thermo-fluidic performance of phase change material slurry and energy transport characteristics

Thermal or cold storage is a promising way to alleviate the peak-valley difference of the electricity load and improve the energy efficiency. The thermal or cold storage by micro-encapsulated phase change material (MPCM) slurry is one of the effective measures to be implemented in waste heat recovery and heat transport system etc. due to its high energy storage density and excellent heat transfer performance. The thermo-fluidic performances of the MPCM slurry flowing through the horizontally circular pipe under constant heat flux are numerically investigated based on the Eulerian-Eulerian approach in the present study. It is found that the numerical results are in good consistence with the experimental results from the aspects of flow and heat transfer. The influences of particle diameter on solid volume fraction distribution, solid velocity distribution and pressure drop of the MPCM slurry are investigated under isothermal condition. And then the temperature distribution of the MPCM slurry and liquid volume fraction of the PCM in MPCM are presented to analyze the heat transfer performance of the MPCM slurry. The particle diameter also imposes significant influences on the heat transfer between the two phases and average heat transfer coefficient of the MPCM slurry. The largest pressure drop and the highest average heat transfer coefficient appear when the particle diameter of the MPCM decreases to 1μm. Further investigations at different Reynolds numbers are carried out to study the variation of local heat transfer coefficient along the pipe. Finally, the comparison of energy transport performances between the MPCM slurry and water are presented based on the ratio of transported heat to pumping power. The MPCM slurry shows better energy transport performance than pure water when the Reynolds number is above 7865.