Large Heat Transfer Coefficient Research Articles

New insights into the thermal behavior and management of thermophotovoltaic systems.

The thermal behavior of a thermophotovoltaic system composed of a metallo-dielectric spectrally selective radiator at high temperature and a GaSb photovoltaic cell in the far field is investigated. Using a coupled radiative, electrical and thermal model, we highlight that, without a large conductive-convective heat transfer coefficient applied to the cell, the rise in temperature of the photovoltaic cell induces dramatic efficiency losses. We then investigate solutions to mitigate thermal effects, such as radiative cooling or the decrease of the emissivity or the temperature of the radiator. Without extending the radiating area beyond that of the cell, gains by radiative cooling are marginal. However, for a given radiator temperature, decreasing its emissivity is beneficial to conversion efficiency and, in cases of limited conductive-convective cooling capacities, even leads to larger electrical power outputs. More importantly, for a realistic radiator structure made of tungsten and hafnium oxide, larger conversion efficiencies are reached with smaller radiator temperatures because thermal losses and thus needs for cooling are less.

Distributed jet array impingement boiling on particle sintered porous surfaces

Due to the excellent heat transfer performance and adaptability in dealing with local hot spots, jet array impingement boiling is promising for the high heat flux cooling in a variety of power electrical and electronic devices, such as high-performance CPU, IGBT, and laser diode. A distributed jet array with effusion ports adjacent to jet holes can extract spent flow immediately to avoid the negative crossflow effect in a normal jet array. Consequently, the heat transfer coefficient (HTC) and critical heat flux (CHF) can be both improved greatly. In order to further improve the heat transfer performance of distributed jet array impingement boiling, enhanced boiling surfaces could be used. Particle sintered porous surfaces have been widely investigated in pool boiling and achieved significant enhancement effect owing to the increased nucleation sites and wetting areas. Although several studies on the sintered porous structure have been done for jet impingement boiling, none is for distributed jet array. In this paper, a pumped two-phase loop was built up and three kinds of porous surfaces were experimentally investigated to enhance the distributed jet array impingement boiling of dielectric coolant, HFE7000. The porous structures were sintered with the copper particles of 50, 100 and 200 μm in diameter on a target surface of 11 mm ×20 mm and had a fixed thickness to particle size ratio of 2.0. A distributed jet array with 10 jet holes and 12 effusion holes was used. The diameters of the jet and effusion holes were 1 mm and 2 mm, respectively, while the distances between jet holes or effusion holes were 5 mm. The volume flow rate through the test section was 0.4 L/min, and the liquid inlet temperature and phase change saturation temperature were controlled at 35 and 60  C, respectively. An experimental apparatus was also established up to compare the wickability of the three porous surfaces. The 100 and 200 μm particles sintered surfaces were proved to have the better wickability. The experimental results turn out that the single-phase heat transfer is significantly improved due to the increased heat transfer area and flow turbulence within the porous structures, and the enhancement ratio increases with the increasing of flow rate. The porous surfaces sintered with 100 and 200 μm particles have close heat transfer performances, which are much better than that sintered with 50 μm particles. As for boiling heat transfer regime, the porous structures can provide more nucleation sites but at the same time will increase the flow resistance of effusion vapor. As a trade-off, the best performance is achieved by 100 μm particle, with its largest heat transfer coefficient upon DNB (departure from nucleate boiling) increased by 61% compared with that of the smooth surface. The wickability of porous layer however has no obvious effect on CHF due to the dominant role of distributed jet impingement on liquid feeding as well as the low surface tension of HFE7000. The maximum enhancement of CHF by 17% is acquired using the 100 μm particle sintered porous surface. Nevertheless, the heat flux range between DNB and CHF is greatly extended for the porous surfaces, which is beneficial to practical applications. A number of spots are found to be randomly distributed on the original smooth particle surfaces after many times of boiling experiments, but seem to have little influence on heat transfer performance. The long-term durability of the enhancement surface however needs further investigation.

Experimental investigation on performances and characteristics of nitrogen-charged cryogenic loop heat pipe with wick-mounted condenser

A cryogenic loop heat pipe (CLHP) provides a highly efficient thermal-conductive link between a cryogenic heat source such as an infrared detector and a cryogenic heat sink like a cryocooler in a spacecraft. The CLHP inherently has a large thermal inertia by the influence of latent heat during phase changes and large heat-transfer coefficients in boiling and condensing of working fluid in cryogenic operating temperatures. This paper presents the experimental results of the nitrogen-charged CLHP which adopts the compact cylindrical wick-mounted condenser and its operational characteristics. The improved compact condenser design was proposed and analyzed comparatively. The recovery of the CLHP to normal state after freezing of the working fluid was verified, and the specific matching characteristic of the heat loads on the primary and secondary evaporators were investigated. The thermal performance and thermal-hydraulic behavior of the CLHP with 680 mm effective heat transport length by stepwise heat-loads were analyzed including state-keeping head load change. The effective thermal conductivity of the CLHP was 112,393 W/(m·K) across 680 mm effective length with 3.1 K temperature difference, which is about 230 times higher than that of a pure copper rod (RRR = 300) in 100 K with 12.17 W heat load.

Enhancing heat rejection from electronic devices with a supercritical carbon dioxide minichannel heat exchanger

The need for high heat flux dissipation in computing centers due to compactness of electronic components has created unforeseen challenges. Due to its large heat transfer coefficients, numerous studies propose the use of phase-change cooling. However, because of the limited operational temperatures allowed within microprocessors, closed cycles often require the employment of refrigerants, which generally present high GWP (Global Warming Potential)/ODP (Ozone Depletion Potential). Here, a s-CO2 (supercritical carbon dioxide) cooling system for thermal management of compact, multi-electronic circuits is investigated. This minichannel heat exchanger cooling unit takes advantage of variations in s-CO2’s properties near the critical point for achieving large thermal conductance and reduced compressing power. It is shown that the cooling unit operating pressure could be controlled to provide maximum cooling rates depending on the thermal load of the electronic circuit. The cooling unit is shown to be compact and to present high heat transfer coefficients (∼104 Wm−2K−1), allowing the electronic component to operate with significant heat transfer rates, and with minimal environmental footprint given the low GWP/ODP of CO2.

Applications of Spray Cooling Technology in Aerospace Field

Spray cooling is an effective heat transfer technology that uses liquid atomization and boiling to to take away the heat from the surface of a cooled object. It has the characteristics of large heat transfer coefficient, good temperature uniformity, small degree of superheat, high critical flux and small circulation flow. It has become a research hotspot of efficient heat and mass transfer technology. The electronic components in the aerospace field are generally in the harsh environment of high heat flux density, high vacuum, microgravity and high radiation. With the increasing use of high-power components in spacecraft, the heat dissipation of spacecraft has become the first problem to be solved in spacecraft design. In response to this problem, the author has studied a large number of spray cooling technologies for aerospace applications, and has certain application value for the research of heat dissipation of high heat flux components in aerospace field.

Direct measurement of the magnetocaloric effect on micrometric Ni-Mn-(In,Sn) ribbons by the mirage effect under pulsed magnetic field

In this work, we report on the direct measurement of the magnetic field induced temperature change in a series of micrometric thick ribbons of Ni-Mn-In-Sn Heusler alloys, performed with an innovative experimental technique based on the thermo-optical “Mirage Effect.” The technique combines very fast measurement time, 1 T pulsed magnetic field in the millisecond range, and contactless temperature detection. These features make the technique ideal for the characterization of thin samples with a thickness down to a few micrometers. In this work, we demonstrate this by directly measuring the magnetocaloric effect (MCE) of micrometric-thick ribbons of Heusler alloys at the Curie transition, which was tuned on a wide temperature range by varying the Sn to In ratio. The direct test of the MCE in thin samples is fundamental for the development of refrigerant elements with a large heat-transfer coefficient and for the design of solid state micro devices for cooling and energy harvesting.

Numerical models to simulate heat and mass transfer at sharp interfaces in nucleate boiling

This work describes a simulation technique for nucleate boiling that considers mass transfer with interfacial temperature gradients and a sharp interface. Interpolation functions estimate the temperature at points located in a direction normal to the interface. A linear temperature profile at the interface interpolates the temperature of the interface-cells. An interface-cells segregation algorithm avoids the interface smearing. Simulations with water at 6.2 K wall superheat show good agreement with the available experimental data on bubble growth rate. Results reveal large heat transfer coefficients over a 200 μm region near the interface. Analysis of the wall shear stress indicates an influence region around the bubble base of two-times the departure bubble diameter.

Investigation of heat transfer from a totally blocked fuel subassembly of fast breeder reactor with 7 and 19 pin bundles

Abstract Liquid metal cooled fast breeder reactor is emerging as an important future power source. It uses sodium as the primary coolant, due to its favourable neutronic properties, high boiling point and large heat transfer coefficient. It is essential to continuously monitor adequacy of fuel pin/fuel subassembly cooling, so that sodium does not reach its boiling point. Sodium boiling within the fuel subassembly leads to large coolant flow reduction (as the density ratio of sodium liquid to sodium vapour is very high), under-cooling of fuel pins and hence damage to fuel subassembly. Development of partial flow blockage in the fuel pin bundle is possible due to the compact nature of fuel pin arrangement within the fuel subassembly, with small hydraulic diameter (typically, ∼3 mm), despite the elaborate design provisions and coolant chemistry control. In the safety analysis, a total flow blockage is usually considered as an upper bound of all the partial blockages. The present work investigates the power level at which liquid sodium reaches its boiling point for different sizes of fuel subassembly. A porous body model has been developed based on local pressure-drop correlations for the axial/cross flow through fuel pin bundles. The model considers the total flow blockage situation in the fuel subassembly wherein the fuel pins and sodium (within the blocked subassembly) are considered as heat generating orthotropic porous medium. The proposed porous body model has been compared with a Computational Fluid Dynamics (CFD) model describing the same blockage condition. The commercial CFD code ANSYS FLUENT 15.0 has been used for the computational purpose. It has been found that the trapped liquid sodium coolant inside the blocked subassembly reaches its boiling point (1153 K at atmospheric pressure) at lower power input per pin in a 19 pin bundle model than that in a 7 pin bundle. This is traced mainly due to the fact that as the number of pins in the subassembly increases, the surface area of the hexcan wall (which is the ultimate heat sink) increases leading to enhanced heat dissipation in large size subassemblies. However, as the number of pins increases, the internal resistance for radial heat transfer increases, leading to increased sodium temperature inside the subassembly. These two competing effects determine the peak sodium temperature in the subassembly. Since the later effect is dominant, the permissible heat generation per pin decreases as the number of fuel pins in the bundle increases.

Thermohydraulic analysis of single phase heat transfer fluids in CSP solar receivers

Abstract Theoretical modelling techniques are used to compare the thermohydraulic performance and thermal storage characteristics of molten salt, liquid sodium, and lead-bismuth in a CSP solar receiver concept. For molten salt, the performance of a number of heat transfer augmentation techniques are also studied. Sodium and lead-bismuth both yield excellent receiver thermal efficiency (max ∼92%), when compared to molten salt (max ∼90%), due to high thermal conductivity values that lead to large heat transfer coefficients. A high pressure drop penalty for lead-bismuth largely offsets its thermal performance gain over molten salt, however sodium retains its advantage as a receiver working fluid with a low pumping parasitic. The implementation of heat transfer enhancement techniques can significantly improve the performance of a molten salt receiver when compared to smooth tube designs. The low specific heat capacity and high unit cost of lead-bismuth is prohibitive towards its use as a storage medium in storage-integrated plant designs, resulting in very high LCOE values. Sodium is the most economically feasible fluid for systems with low storage (

Experimental investigation and theoretical analysis of oil circulation rates in ejector cooling cycles

In this study, the influence of compressor speed, ejector motive nozzle needle position and evaporator inlet metering valve opening on the oil circulation rates (OCRs) of an R744 transcritical standard ejector cycle was experimentally investigated. Significantly higher OCR (∼10%) was observed at the evaporator inlet of the ejector cycle than that at the high pressure side (∼1%) measured in the same cycle under the same conditions. It has been observed that evaporator OCR was increased with increasing compressor speed. When the motive nozzle needle moved towards the nozzle throat, both compressor discharge flow rate and evaporator OCR were observed to be significantly lowered. As the evaporator inlet metering valve opening was adjusted, the compressor mass flow rate did not vary significantly while the evaporator mass flow rate decreased with decreasing metering valve opening. The evaporator OCR decreased from 6.5% to 2.2% as the metering valve opening varied from 86% to 27%. High evaporator OCR results in large evaporator pressure drop and low heat transfer coefficient. In addition to the standard ejector cycle, several alternative ejector cycles were theoretically analyzed to see if there is similar problem of high OCR in the evaporator. In ejector liquid recirculation cycle and multi-stage multi-ejector supermarket refrigeration cycle, similar high OCR problem in the evaporator may exist, while in two evaporator ejector cycle, evaporator OCR is equal to compressor OCR at steady state.

Ultrafast synthesis of TS-1 without extraframework titanium species in a continuous flow system

When preparing porous crystalline materials, low energy utilization and long crystallization times (∼48 h) as well as the instability of the products makes it difficult for traditional hydrothermal technology using batch reactors to meet the ever increasing environmental requirements of the chemical industry. However, rapid crystallization can be realized when using a continuous flow system such as a tubular reactor due to large heat transfer coefficient. Herein, we demonstrate the rapid continuous hydrothermal synthesis of nanoparticles of TS-1 with a tubular reactor. The particles have MFI structures and a very narrow size distribution of 100–150 nm after an 8 min synthetic step in tubes with an inner diameter of 2 mm and an external diameter of 3 mm. The results show that a faster nucleation rate improved the proportion of titanium entering into the silicon skeleton, with almost all Ti species existing in the tetrahedral coordination state at a grain size of 42 nm. This simple and intrinsically safe route allows good quality control and high productivity with very little equipment, and allows for easy scale-up. This first case of fast synthesis of TS-1 may provide some valuable references for the development of green synthesis of much more crystalline materials.

Nanoparticle migration and natural convection heat transfer of Cu-water nanofluid inside a porous undulant-wall enclosure using LTNE and two-phase model

Using Buongiorno's model, the natural convection of Cu-water nanofluid flowing within a wavy wall porous enclosure at the presence of a cylindrical heater was numerically investigated. The problem is considered such that there is no local thermal equilibrium between two phases of porous media. The effects of Rayleigh number (103≤Ra≤106), the Lewis number (1≤Le≤50), Brownian motion parameter (0.1≤Nb≤0.5), the thermophoresis parameter (0.1≤Nt≤0.5), the buoyancy ratio (0.1≤Nr≤0.5), the Darcy number (10−5≤Da≤10−1), porosity (0.1≤ε≤0.9), the ratio of the thermal conductivity of the nanofluid to the solid phase (0.1≤γs≤10), coefficient of heat transfer at the interface (1≤Nhs≤1000) on hydrodynamic and thermal parameter have been studied. Streamlines, distribution of temperature and concentration of nanoparticles are presented and discussed. Increasing in the Rayleigh, the Darcy, the Lewis numbers, the thermal conductivity ratio and the Brownian parameter of the nanoparticle motions homogenizes the nanofluid. Meanwhile, the homogeneity is reduced as thermophoresis is increased. The results show that the thermal equilibrium model for porous media is applicable for the cases with large heat transfer coefficient at the interface between the solid matrix and the nanofluid. Moreover, the average Nusselt number of both porous phases' increases with the Brownian parameter, while the Nusselt number is reduced as the thermophoresis is increased.

Thermal and hydraulic numerical study for a novel multi tubes in tube helically coiled heat exchangers: Effects of operating/geometric parameters

Compactness with high performance heat exchangers are main challenges in a lot of engineering applications. Thus, this research reports CFD simulation of a novel MTTHC (multi tubes in tube helically coiled) heat exchanger using ANSYS-FLUENT 14.5. The aim of the work is to investigate the thermal and hydraulic performance of the MTTHC for turbulent flow. The effects of the operating and geometrical parameters of the coil on the cold/hot water Nusselt numbers, heat transfer coefficients, pumping power, effectiveness, and thermal-hydraulic index are studied and presented. The results show that, the largest heat transfer coefficient is found at N = 3 and β = 0° & 90°, and the pumping power (P) rises with ≅20 times if N changed from 1 to 5 at any β. Moreover, the effectiveness of the coil (ε) has the largest values at β = 0° & 90° and N=3, and it enhances with 8.5%, 9% and 7% if N increased from 1 to 3 at β = 0°, 45° and 90°, respectively. In addition, thermal-hydraulic index (ξ) improves with 5%, 8% and 6% if N increased from 1, to 3 at β = 0°, 45° and 90°, respectively. Finally, Numerical correlations for P, ε and ξ are correlated and presented within reasonable errors.

Irradiation Target Cooling Using Circular/Slot Air Jet

ABSTRACTTo study the effect of irradiation on materials, sample coupons are irradiated in cyclotron facilities. During the irradiation process, these samples produce significant heat. This heat needs to be continuously removed from the samples in order to avoid melting of the samples as well as to keep the samples at a particular temperature during irradiation. The area available for heat transfer is limited due to small size of the samples. To increase the heat transfer rate, jet cooling is used as it provides large heat transfer co-efficient. To understand the heat transfer characteristics of jet cooling under these conditions, experiments have been carried out. Two inclined jets hitting on both sides of the target plate give maximum cooling and uniform temperature distribution. This paper gives the details of the numerical and experimental studies carried out and the discussions about the results obtained.

Quantitative prediction of critical heat flux initiation in pool and flow boiling

Boiling is a very efficient heat transfer mechanism with a large heat transfer coefficient and it is widely found in industrial systems. However, boiling heat transfer is limited by the critical heat flux (CHF), also termed as boiling crisis. It leads to a rapid decrease of the heat transfer coefficient in temperature controlled heat transfer or to a significant jump in heater surface temperature in power controlled heat transfer cases. While the earlier effect clearly lowers efficiency the latter may even jeopardize safety. A clear understanding of the basic mechanisms leading to CHF is still lacking. In this paper a new model of priori critical heat flux (CHF-) is derived from the bubble dynamics of nucleate boiling. It holds for pool boiling and forced convective boiling and incorporates a mutual effect model and a shear stress model. The comparison between predicted and experimental results under different thermal hydraulic conditions shows a good agreement. The model is capable to explain the initiating mechanism of the boiling crisis and impacts from different variables. It can be also implemented as a sub-model in CFD codes.

Superhydrophobic Si nanowires for enhanced condensation heat transfer

Condensation is an essential process in various industrial systems. Enhancing condensation by employing superhydrophobic (SHB) surfaces had drawn significant attention in recent years because of the emerging technology for surface engineering. However, the efficacy of SHB surfaces in condensation is controversy in the literature. The observed deteriorated heat transfer on SHB in condensation is presumably a result of highly pinned Wenzel droplets or flooding formed on the SHB surfaces. Si nanowire (SiNW) array-coated surface which can simultaneously provide a large number of nucleation sites and prevent condensate from penetrating into the nano-structure is a promising candidate for enhancing condensation. In this work, heat transfer on the SHB SiNW surface was investigated. At low subcooling, jumping of liquid droplets accompanied with a high droplet departure frequency resulted in a large heat transfer coefficient (HTC) of 88±16kW/m2K on the SHB surface. This value is one of the highest reported condensation HTCs in the literature. It was 155% and 87% higher than those on the plain hydrophilic and hydrophobic surfaces, respectively. Heat transfer decreased with the rise of subcooling due to an increased condensate surface coverage ratio. However, condensate can still be rapidly shed away from the SHB SiNW surface at high subcooling, which render the comparatively larger HTC of 18.6±2kW/m2K on the SHB SiNW surface as opposed to plain hydrophobic and hydrophilic surfaces. It was evidenced that SHB surface could have a superior heat and mass transfer performance than hydrophobic surface provided that the liquid droplets on the SHB could be shed away efficiently.

Steady state and transient analytical modeling of non-uniform convective cooling of a microprocessor chip due to jet impingement

Heat removal from microprocessor chips with multiple regions of dynamic heat generation remains a critical technological challenge. Excessive temperature rise is undesirable for performance as well as reliability. Jet impingement cooling has been widely investigated as a potential thermal management technique due to the capability of localized cooling and of dynamically following the heat generation distribution. A jet offers large local convective heat transfer coefficient, for which theoretical models and correlations have been proposed for a variety of scenarios. However, not much work exists on using this information to determine the resulting temperature distribution. This paper addresses this need by developing analytical steady state and transient heat transfer models that account for the spatial variation in convective heat transfer coefficient and for spatially non-uniform heat flux. The solution is derived in the form of an infinite series, the coefficients of which are determined by solving a set of algebraic equations. Temperature rise predicted by the models are found to be in excellent agreement with finite-element simulations, while offering faster computation time and easier integration with design and performance optimization tools used in microelectronics. The analytical model is used for predicting temperature rise in a variety of scenarios to examine interesting optimization problems such as the cooling of multiple hotspots with a single jet, determining the optimal location of a jet, etc. Results presented here may facilitate improved thermal design and real-time performance optimization of microprocessor chips.

Cooling of small size irradiation specimens using impinging jets

To study the effect of irradiation on materials, sample coupons are irradiated in cyclotron facilities. During the irradiation process, these samples produce significant amount of heat. This heat needs to be continuously removed from the samples in order to avoid melting of the samples as well as to keep the samples at a particular temperature during the irradiation process. The area available for heat removal is limited due to the small size of the samples. To increase the heat removal rate, jet cooling is employed as it provides large heat transfer co-efficient. To understand the heat transfer characteristics of jet cooling under these conditions, experiments have been carried out. Electric Joule heating is adopted to simulate irradiation heat in stainless steel samples. Circular / slot nozzles are used to create air jet. From the study the values of the parameters correspond to the maximum heat removal rate are found out. The results are also compared with empirical correlations from the literature.

CFD SIMULATION OF UPWARD SUBCOOLED BOILING FLOW OF FREON R12

Subcooled flow boiling under forced convection occurs in many industrial applications of purpose to maximize heat removal from the heat source by the very large heat transfer coefficient. This work deals with CFD simulations of the subcooled flow boiling of refrigerant R12 solved by code ANSYS FLUENT r16. The main objective of this paper is verification of used numerical settings on relevant experiments performed on DEBORA test facility. Also comparisons with previously provided simulation on NRI Rez are presented. Data outputs from this work are basis to subsequent calculations of steam-water mixture cooling of Pb-Li eutectic.

Effects of Operating Temperature on the Heat Transfer Characteristics of Photovoltaic Systems in the Upper Midwest

This paper presents the heat transfer characteristics of a stationary PV system and a dual-axis tracking PV system installed in the Upper Midwest, U.S. Because past solar research has focused on the warmer, sunnier Southwest, a need exists for solar research that focuses on this more-populated and colder Upper Midwest region. Meteorological and PV experimental data were collected and analyzed for the two systems over a one-year period. At solar irradiance levels larger than 120 W/m2, the array temperatures of the dual-axis tracking PV system were found to be lower than those of the stationary system by 1.8 °C, which is a strong evidence of the different heat transfer trends for both systems. The hourly averaged heat transfer coefficients for the experiment year were found to be 20.8 and 29.4 W/m2 °C for the stationary and tracking systems, respectively. The larger heat transfer coefficient of the dual-axis tracking system can be explained by the larger area per unit PV module exposed to the ambient compared to the stationary system. The experimental temperature coefficients for power at a solar irradiance level of 1000 W/m2 were −0.30% and −0.38%/ °C for the stationary and dual-axis tracking systems, respectively. These values are lower than the manufacturer's specified value −0.5/ °C. Simulations suggest that annual conversion efficiencies could potentially be increased by approximately 4.3% and 4.6%, respectively, if they were operated at lower temperatures.