Moderate Heat Fluxes Research Articles

Qualification of W heavy alloys as plasma facing material

Highly loaded divertor regions in fusion experiments are exposed to high particle and power fluxes. Therefore, tungsten with its excellent thermal and physical properties is a preferred plasma facing material for this application. For divertor areas with moderate steady-state heat flux up to 10 MW/m2 the excellent high-temperature resistance of W is not absolutely necessary. 95–97 % W heavy alloys containing Ni, Cu or Fe could be used from the aspects of plasma wall interaction and their thermomechanical properties. In this study we present results of thermo-mechanical characterisations like measurement of thermal conductivity and tensile tests at elevated temperatures as well as results of high heat flux performance tests at adiabatic and steady-state loading. Finally, we discuss results of deuterium retention measurements.

Experimental investigation on boiling heat transfer performance of fractal microchannels for high heat dissipation applications

This study proposes novel fractal microchannels (FMCs) manufactured by additive manufacturing for direct cooling systems with high heat dissipation. The boiling heat transfer performance of water in the FMC was experimentally investigated under various operating conditions. Furthermore, a comparative heat transfer performance evaluation between the FMC and conventional microchannels (CMCs) was conducted. The critical heat flux of water in the FMC was 11 % higher than that in the CMC due to enhanced boiling heat transfer. In the low heat flux region, the boiling heat transfer coefficient (BHTC) of water in the FMC increased as the mass flow rate increased owing to the liquid replenishing and rewetting effects. However, in the moderate and high heat flux regions, the effect of the mass flow rate was not significant owing to the dominance of nucleate boiling. The BHTC of water in the FMC increased as the water temperature decreased due to the increased rewetting frequency. This study provides useful information for improving the design of direct cooling channels and for understanding the fundamental boiling heat transfer characteristics in FMCs. Furthermore, the proposed FMC can be applied in fields requiring high heat dissipation, such as electronic devices, power plants, and hot stamping.

Investigations on Phthalonitrile – Cyanate ester blends and their light weight composites; Synthesis, thermal, mechanical and ablative characteristics

Newer high performing polymer matrix resins are highly demanded given the rapid development trend in aero-space exploration sector. The present work focuses on synthesis and performance evaluation of Phthalonitrile- Cyanate ester blend as matrix resin for ablative composite applications. Incorporation of Cyanate ester facilitated early cure initiation (by about 35 °C) of Phthalonitriles without comprising thermal stability, a novel strategy that improves the sluggish cure profile of phthalonitriles. Low density (0.5g/cc) syntactic foam composites prepared from the blend exhibited low Coefficient of Thermal Expansion (CTE) (1.3 × 10−5 – 1.7 × 10−5/°C), high specific heat (0.32–0.38 J/g/°C at 80 °C) and low thermal conductivity (0.11–0.13 W/mK at 80 °C). Exposure of the composites to moderate re-entry heat flux conditions using radiant heating facility (∼55W/cm2, 1200s) and Plasma Arc jet (70&125 W/cm2, 50s) resulted in excellent surface uniformity with minimal of mass loss (4.77–5.77%) and excellent heat of ablation (8000–12000 cal/g) with reduced back-wall temperature (230–260 °C). The composites possessed very good mechanical properties such as Flexural strength of 5–6.5 MPa and compressive strength of 5–9 MPa. 1:1 combination of pyrolyzed system possessed high degree of graphitization. The promising results confirm that the new matrix blend can be a potential replacement for conventionally used phenolics as ablative Thermal Protection System (TPS) matrix.

Experimental investigation of buoyancy effects on the heat transfer of supercritical carbon dioxide in a vertical tube

This study experimentally investigates the effect of buoyancy on the heat transfer characteristics of supercritical CO2 in a vertical tube of 7.74 mm inner diameter. The experiments are performed for inlet temperatures ranging from 14 to 33 °C, pressure from 7.8 to 10 MPa, heat flux from 29.0 to 60.3 kW/m2, and mass flux from 138.3 to 512.1 kg/m2·s. The influences of heat flux, mass flux, pressure, inlet temperature, and flow direction on the heat transfer are analyzed with an emphasis on the buoyancy effect. The results show there is only one wall temperature peak at moderate heat flux and two peaks at high heat flux. The heat transfer under different mass fluxes is completely different due to the buoyancy effect. Current experimental data differ significantly from widely used empirical correlations. Therefore, a new correlation with a buoyancy parameter is developed, which provides better prediction accuracy of the Nussult number.

Heat transfer coefficient and pressure drop of water flow boiling in porous open microchannels heat sink

• Porous open microchannels (POM) heat sink was proposed and fabricated. • Heat transfer coefficient was greatly improved by porous wall at high heat fluxes. • Periodic slug-stratified flow was newly observed in POM heat sink. • No local dryout was noted due to the enhanced capillary actions in porous wall. • Two-phase flow could maintain stable under all operating conditions tested. The open microchannel is attracting more attention due to the reduced pressure drop and the improved flow stability, but the local dryout easily occurs at the downstream of the microchannels and subsequently deteriorates the flow boiling heat transfer performance. In this study, two open microchannels heat sinks were fabricated utilizing solid copper substrate and the porous substrate sintered by copper powder particles, respectively. Flow boiling experiments were conducted using deionized water as the working fluid with the inlet temperate of 87 °C at different mass fluxes ranging from 151.5 to 551.1 kg/m 2 s over a heat flux range of 29.6-1871.1 kW/m 2 in the porous open microchannels (POM) and the solid copper open microchannels (SOM). Experimental results showed that the heat transfer coefficients (HTC) were greatly improved in POM compared with that in SOM at moderate (300 kW/m 2 < q eff < 600 kW/m 2 ) and high ( q eff > 600 kW/m 2 ) heat fluxes. No local dryout was observed even at the highest heat fluxes in POM heat sink. Therefore, the flow boiling HTC in POM no longer deteriorated with increment in heat flux at high heat flux region, unlike in SOM. A new periodic flow pattern named slug-stratified flow was found in POM heat sink during which the slug flow and the stratified flow alternated temporally and spatially. Besides, although the two-phase pressure drop of POM was larger than that of SOM due to the increased friction and acceleration loss, the flow could still maintain stable under all operating conditions tested here.

Flow Boiling Heat Transfer of Grooved Copper Foam with Open Gap

Introduction. Copper foam material has various advantages. It has been proved effective in enhanced boiling heat transfer, but also increases pump power consumption. Grooved copper foam is a solution to achieve good balance between boiling heat transfer characteristics and pump power consumption. Material and Methods. Grooveless and grooved copper foam in open space was studied. Copper foam specifications comprised the combination of porosities of 70, 80 and 90%, and pore densities of 90 and 110 PPI. The grooved copper foams have two specifications: 11 and 17 grooves. The corresponding rib widths are 2 and 1 mm, with groove depth 2.9 mm and width 0.6 mm. The flow boiling experimental system of copper foam sample includes four parts: a heating water reservoir, pump, a test section, and a data acquisition system. In the test section, liquid water turns into vapor and carries the heat away from a copper block surface, and then vapor condenses into liquid water in the terminal reservoir. Results. Grooved copper foam samples presented significantly higher efficiency than grooveless ones. Grooved copper foams can increase the critical heat flux and heat transfer coefficient, compared with grooveless ones. Seventeen-grooved samples showed more excellent performance than 11-grooved ones. Visual observation disclosed that the stratified flow pattern dominated in moderate and high heat flux for grooved copper foam with open space. Covering vapor mass was more effective to be formed above 17-grooved samples, compared with 11-grooved ones. It indicated more vigorous boiling behavior occurs in 17-grooved sample. Discussion and Conclusion. The number of grooves has a significant impact on boiling heat transfer. Grooved copper foam samples present a significantly higher critical heat flux and heat transfer coefficient. Structural parameters such as porosity and pore density, play a relatively secondly role in heat transfer argumentation. Visual observation shows there exists a cyclic alternation of flow patterns: bubbly flow, annular flow and mass vapor formation for grooved samples. Forming vapor mass is more effective to be formed in 17-grooved samples, compared to 11-grooved ones. It indicates more vigorous boiling behavior occurs in 17-grooved samples.

Experimental investigations of pump‐driven closed‐loop thermosyphon system

AbstractThe thermal management of the compact electronic system is one of the major challenges in the present power electronics and computational industries. The present paper investigates the effects of a pump‐driven flow on the thermal performance of a closed‐loop thermosyphon (CLT) system. This paper discusses the effect of pump‐driven flow on the thermal performance of CLT. In this study, the experimentation is carried out on the water‐charged pump‐driven closed‐loop thermosyphon (PDLT) with different heat inputs, filling ratios (FR), and adiabatic lengths to understand the effects of these parameters on the thermal performance of the system. The results indicate that the heat transfer performance of CLT is improved using pump‐driven flow in the PDLT and the minimum thermal resistance (0.035 k/W) is obtained at FR = 0.6 and 3 kW heat input. It is also noticed that the thermal resistance of PDLT is up to 35% higher than CLT at FR = 0.6, 0.5 kW heat input, and 500 mm adiabatic length. The unstable geyser boiling phenomenon is eliminated from the system at the adiabatic lengths of 200 and 500 mm. However, for long adiabatic length (800 mm), the geyser boiling occurs at a higher FR (FR = 0.6) and moderate heat flux (0.5 kW).

Thermosyphon-assisted cooling system working in the moderate heat flux range

• Thermosyphon-assisted cooling system was used to remove moderate heat fluxes; • 25% filling of a thermosyphon with ethanol provides the best cooling efficiency; • A thermally insulated thermosyphon can be used, but with mixed or forced convection; • The efficiency of thermally insulated thermosyphon is lower than that of uninsulated. Ensuring the permissible temperature regime of heat-loaded and energy-saturated equipment is still an urgent problem in heat power engineering and thermal physics. In this work, it is demonstrated that thermosyphons without a boiling coolant on their lower lid can be used in practice as self-regulating heat exchangers ensuring the permissible temperature regime of heat-loaded and energy-saturated equipment. The experiments showed that heat flux in the range of 0.18–2.60 kW·m −2 can be removed from engineering heated surfaces of equipment by a self-regulating thermosyphon-assisted cooling system (TCS) with intense evaporation of a coolant (without boiling) on its lower lid and air cooling of the condenser due to natural convection. Using modern tools for registration of thermophysical and hydrodynamic processes, the effect of the coolant type, the filling ratio of evaporator and the presence of thermal insulation on the side surface of a TCS on its efficiency was studied. To increase the efficiency of the thermally insulated TCS, it is advisable to create mixed or forced convection above the upper lid with the heat transfer coefficient of 22–53 W·m 2 ·K −1 . It was hypothesized that the heat transfer intensity in the TCS characterizing by the temperature distributions and evaporation rates depends on heat transfer and convection in the liquid layer on the lower lid of the TCS.

Effect of R-600a and distilled water on nucleate pool boiling characteristics from horizontal tubes at low and moderate heat flux

Effect of R-600a and distilled water on nucleate pool boiling characteristics from horizontal tubes at low and moderate heat flux

Experimental Study of Halloysite Nanofluids in Pool Boiling Heat Transfer.

Halloysite nanotube (HNT) which is cheap, natural, and easily accessible 1D clay, can be used in many applications, particularly heat transfer enhancement. The aim of this research is to study experimentally the pool boiling heat transfer (PBHT) performance of novel halloysite nanofluids at atmospheric pressure condition from typical horizontal heater. The nanofluids are prepared from halloysite nanotubes (HNTs) nanomaterials-based deionized water (DI water) with the presence of sodium hydroxide (NaOH) solution to control pH = 12 to obtain stable nanofluid. The nanofluids were prepared with dilute volume concentrations of 0.01–0.5 vol%. The performance of PBHT is studied via pool boiling curve and pool boiling heat transfer coefficient (PBHTC) from the typical heater which is the copper horizontal tube with a thickness of 1 mm and a diameter of 22 mm. The temperatures of the heated tube surface are measured to obtain the PBHTC. The results show an improvement of PBHTC for halloysite nanofluids compared to the base fluid. At 0.05 vol% concentration, HNT nanofluid has the best enhancement of 5.8% at moderate heat flux (HF). This indicates that HNT is a potential material in heat transfer applications.

Heat Transfer Performance of R-1234ze(E) with the Effect of High-Viscosity POE Oil on Enhanced GEWA-B5H Tube

In this study, the heat transfer performance of high-viscosity polyol ester (POE) oil POEA-220 (220 cSt) with low-GWP (global warming potential) refrigerant R-1234ze(E) on enhanced GEWA-B5H tube was investigated at saturation temperatures of 10 °C, 0 °C, and −6 °C. The mass fraction of oil varied from 0.25% to 10%, and all the nucleate pool boiling data were measured at heat fluxes ranging from 10 kW/m2 to 90 kW/m2. The experimental results showed that the heat transfer performance of the R-1234ze(E)/POEA-220 mixtures were superior to the R-1234ze(E)/POEA-68 mixtures. At saturation temperatures of 0 °C and −6 °C, even a 10% mass fraction of the POEA-220 oil showed an enhancement in the HTC (heat transfer coefficient) compared to the pure refrigerant in the moderate heat flux range. On the other hand, for the R-1234ze(E)/POEA-68 mixtures, a 5% mass fraction of oil showed no enhancement in the HTC compared to pure refrigerant at the same saturation temperature. Moreover, at low saturation temperatures (0 °C and −6 °C), the enhancement in the HTC decreased with increasing mass fraction of low-viscosity oil POEA-68, whereas high-viscosity oil POEA-220 showed the highest enhancement in the HTC for a 5% mass fraction of oil at −6 °C saturation temperature compared to the pure refrigerant. The results indicate that for nucleate boiling, the effect of oil viscosity on heat transfer performance is negligible if it contains comparatively high thermal conductivity and low surface tension. In addition, the effect of surface aging on heat transfer performance for the GEAW-B5H tube with pure refrigerant was also reported.

Pyrolysis and autoignition behaviors of oriented strand board under power-law radiation

This contribution addresses a multi-component high order parallel reaction scheme developed for pyrolysis of Oriented Strand Board (OSB), a typical engineered wood product, and its application in estimating autoignition behaviors under power-law heat flux (HF). Thermogravimetric analysis tests were conducted first to parameterize the pyrolysis model by model fitting method. Subsequently, gram-scale autoignition experiments, using five power-law HFs, were implemented in a newly designed apparatus. Thermodynamics of OSB were determined by inverse modelling combining an improved numerical model and the measured surface temperatures and mass loss rates under a moderate HF. The extrapolation capability of the developed model was verified by simulating the remaining experimental measurements at alternative heating scenarios. Both critical temperature and critical mass flux were employed in predicting autoignition times. The results show that the developed pyrolysis model accurately captures the measured mass and mass loss rate collected in TGA tests. Meanwhile, relatively good agreement was found between the simulated and measured surface temperatures and mass loss rates in bench-scale tests despite some minor divergence due to the observed cracks of generated char layer. Furthermore, the uncertainties of the attained kinetic and thermodynamic parameters were quantitatively evaluated by parametric study.

Heat transfer and hysteresis characteristics in an elbow thermosyphon

Measurements were performed to characterize the performance of a grooved copper–water elbow thermosyphon with a fluid loading of 30%, typical of thermosyphons used for power electronic devices. The evaporator section was vertical, while the condenser section was inclined at an angle 8°to the horizontal. The performance of the device, characterized for incrementally increasing and decreasing heat transfer rates, shows a significant hysteresis in the evaporator performance at moderate heat transfer rates or fluxes (between 2 and 8 W/cm2). The difference was due to changes in the upper portion or film region of the evaporator. Transients of the temperature suggest a change to a more dynamic flow regime in the evaporator at a heat flux of approximately 6 W/cm2. The transition occurred at similar heat fluxes for both the increasing and decreasing heat transfer rates. The improved performance in the upper or film region was observed after the dynamic regimes, suggesting it may be due to the wetting of the internal structure during this process. The effect was reduced with increasing operating temperature.

Pyrolysis and autoignition behaviors of beech wood coated with an acrylic-based waterborne layer

Pyrolysis and autoignition of beech wood covered with an acrylic-based waterborne coating layer are experimentally and numerically examined. Thermogravimetric Analysis (TGA) tests were conducted first for wood and coating to derive the kinetic parameters combining a numerical model, Genetic Algorithm (GA) and Particle Swarm Optimization (PSO). Subsequently, bench scale autoignition tests of thermally thick wood and coating-wood composite with varying coating thickness were performed under four radiative heat fluxes. Thermal conductivities and heat capacities of wood, coating and yielded residue were determined by inversely modelling the measured surface and 3 mm in-depth temperatures at a moderate heat flux. Reliability of the fully parameterized model was validated by simulating the collected surface temperatures and mass loss rates at the remaining heating conditions. Meanwhile, the recorded autoignition times were predicted by the model using the measured critical temperature and critical mass flux. The results show that both GA and PSO yield identical accuracy in fitting the TGA measurements, and their average results are adopted. The parameterized model successfully captures the measured surface temperatures and mass loss rates before ignition despite some minor deviations. Application of the coating considerably improves the fire resistance of wood by increasing the critical heat flux, critical temperature, critical mass flux and ignition time. The measured autoignition times are relatively well estimated, and an approximate linear correlation exists between ignition time and coating thickness.

Effect of wettability on nucleate pool boiling heat transfer of a low surface tension fluid outside horizontal finned tubes

Nucleate pool boiling heat transfer of a low surface tension fluid (R134a) is tested outside the surfaces with polymer coating. Nanoscale Parylene films are coated on the surfaces with Plasma Enhanced Chemical Vapor Deposition(PECVD) method. The thickness of the coating is about 10 nm. Heat flow rate intensity varies in the scope of 15–150 kW/m2. Saturation temperatures are 6, 10 and 16 °C, respectively. According the investigation, for the smooth tube, it is found that the roughness of the surface reduced after the coating and the pool boiling heat transfer performance is also deteriorated. The boiling heat transfer can be enhanced to a certain degree for two finned structures. For the saw-teeth tube, the heat transfer coefficient(HTC) can be 1.1–1.3 times higher than that without coating. For the reentrant cavity tube, at lower heat flux, the effect of coating on the performance is insignificant. While, the heat transfer can be enhanced up to a factor of 1.2 at higher heat flux. In general, the effect of the Parylene polymer coating on the pool boiling of the low surface tension fluid R134a are insignificant at the moderate heat flux

Flow-Controlled Spray Cooling Approaches for Dynamic Thermal Management

Abstract Two-phase spray cooling is considered as one of the most promising thermal management techniques characterized by high heat transfer coefficient (HTC) and critical heat flux (CHF), as well as near-uniform temperatures on target surface. Normally, spray cooling systems feature steady flow rate and continuous spray, and are designed to satisfy the maximum expected heat load. However, if the heat load varies due to operating conditions, such as cold starts and pulsing power cycles, a spray cooling system uses excessive coolant most of the time, and causes low cooling efficiencies because of lower utilization of phase-change heat transfer and higher pumping power. This study aimed to investigate variable and intermittent flow spray cooling characteristics for dynamic thermal management. Variable flow spray cooling scheme requires control of pump input voltage (or speed), and intermittent flow spray cooling scheme requires control of solenoid valve duty cycle and frequency. Tests were conducted on a closed-cycle system with HFE-7100 dielectric coolant at varying liquid flow rate and subcooling conditions, and using smooth and enhanced heat transfer surfaces. Results, compared to a baseline performance from steady flow case, indicated that the variable flow spray cooling conditions achieve similar cooling performance at low and moderate heat fluxes, while the intermittent flow spray conditions result in high temperature penalty and much lower CHF.

Thermal properties of U-Mo alloys irradiated under high fission power density

A variety of physical and thermal property measurements made on irradiated U-Mo alloy monolithic fuel samples with a Zr diffusion barrier and clad in Al alloy 6061 are reported. These properties form the basis for additional investigations in thermal behavior of U-Mo fuel under high performance research reactor irradiation conditions. Measurements were performed on samples harvested from fuel plates irradiated under high fission power density and surface heat flux and complement previous measurements on samples subjected to moderate fission power density and surface heat flux. Sample measurements reported in this work represented fission density ranging from 2.42 – 4.81 × 1027 fissions•m−3 (20.8 – 36.5 235U depletion) and average plate surface temperatures from 355 – 414 K. Specific heat capacity of irradiated monolithic U-Mo increases with increasing temperature and fission density, but carries large uncertainties associated with thickness measurements used to determine the mass fraction of the layer constituents for extraction of the U-Mo specific heat capacity, specifically the very thin and nonuniform Zr diffusion barrier. Density of irradiated monolithic U-Mo decreases with increasing fission density resulting from fission gas swelling and porosity formation. Thermal diffusivity of irradiated monolithic U-Mo increased with increasing temperature and decreased with increasing fission density. Thermal conductivity of irradiated monolithic U-Mo increased with increasing temperature, and in general, appeared to decrease and become less sensitive to temperature with increasing fission density. Thermal conductivity measurements compared to a semi-empirical model agreed within ±30%. It is suspected that the main driver in deviation between model calculations and measurements is the fission gas swelling determination, which does not account for fission power density or irradiation temperature.

Experimental study of flow boiling performance of open-ring pin fin microchannels

A type of open-ring pin fin microchannels (ORPFM) was proposed and developed for advanced microchannel heat sinks. They consist of an internal cavity for bubble nucleation and two inner small and outside large rings with separated and converged flow passages. Two arrangements of open-ring pin fins, inline and staggered ones, were prepared and compared for optimization. Flow boiling experiments were performed to assess the two-phase boiling performance of both inline and staggered ORPFM. Tests were conducted using subcooled deionized water with an inlet subcooling of 10°C and mass fluxes of 200 kg/m2·s and 300 kg/m2·s. Two-phase boiling heat transfer performance, pressure drop and two-phase flow instabilities of the ORPFM were systematically explored. Experimental results indicated that both inline and staggered ORPFM samples presented a small wall superheat of 1-2°C for onset of nucleate boiling (ONB). Nucleate boiling, both nucleate boiling and thin film evaporation, and convective boiling in turn dominated with increasing heat flux and vapor quality for ORPFM. The inline ORPFM generally presented a 10% to 80% augmentation in boiling heat transfer at moderate and high heat fluxes, a 2%-20% smaller pressure drop, and more stable flow boiling process with a mitigation of two-phase flow instabilities than the staggered one. Therefore, the inline ORPFM seemed to be more favorable for high heat flux dissipation of advanced microchannel heat sinks.

Heat Transfer in Supercritical Steam Flowing Through Spiral Tubes

Abstract We investigate heat transfer in supercritical steam flowing in a spiral tube by conducting three-dimensional numerical simulations. The current numerical solver has been validated with the existing experimental results, and simulations are performed by varying different geometric parameters of a spiral tube. The flow dynamics and heat transfer in a spiral tube are compared against those in a straight tube. For the parameters range considered in the present study, it is found that the heat transfer coefficient (HTC) in the spiral tube is 29% higher than that in the case of a straight tube for the same flow and thermal conditions. Our results indicate that the tangential velocity component resulting due to the spiraling effect of the steam is the primary reason for the enhancement of the HTC value. It is observed that while the HTC in a spiral tube is inversely related to the spiral diameter, it does not exhibit a strong relationship with the spiral pitch. Moreover, three existing heat transfer correlations are evaluated under the spiral flow condition and it is observed that none of them can calculate the HTC value accurately in spiral tubes. Using the Buckingham π-theorem, three modified correlations are proposed for the low, moderate, and high heat flux regimes, which accurately predict the wall temperature and HTC of supercritical steam in spiral tubes in all the heat flux regimes. The correlations have an error band of less than ±20%.

Boiling Heat Transfer Performance of Parallel Porous Microchannels

Flow boiling in microporous layers has attracted a great deal of attention in the enhanced heat transfer field due to its high heat dissipation potential. In this study, flow boiling experiments were performed on both porous microchannels and a copper-based microchannel, using water as the coolant. As the heat flux was less than 80 W/cm2, the porous microchannels presented significantly higher boiling heat transfer coefficients than the copper-based microchannel. This was closely associated with the promotion of the nucleation site density of the porous coating. With the further increase in heat flux, the heat transfer coefficients of the porous microchannels were close to those of the copper-based sample. The boiling process in the porous microchannel was found to be dominated by the nucleate boiling mechanism from low to moderate heat flux (&lt;80 W/cm2).This switched to the convection boiling mode at high heat flux. The porous samples were able to mitigate flow instability greatly. A visual observation revealed that porous microchannels could suppress the flow fluctuation due to the establishment of a stable nucleate boiling process. Porous microchannels showed no advantage over the copper-based sample in the critical heat flux. The optimal thickness-to-particle-size ratio (δ/d) for the porous microchannel was confirmed to be between 2–5. In this range, the maximum enhanced effect on boiling heat transfer could be achieved.