Simplified model to analyze the thermal behavior of a non-evacuated receiver tube integrated with a Compound Parabolic Concentrator (CPC) for Linear Fresnel Reflector systems is presented. Unlike evacuated receivers, CPC-based systems avoid high manufacturing costs and vacuum maintenance. An experimental investigation is conducted for a non-evacuated receiver to measure surface temperatures of the absorber tube, glass tube, CPC, and Flat Glass Cover (FGC) under steady conditions. The absorber temperature is varied between 323-493 K. The overall heat loss coefficients (U1) are estimated for five configurations: (1) Absorber tube, glass tube only (2) Absorber tube, glass tube, CPC without insulation and without FGC (3) Absorber tube, glass tube, CPC with insulation and without FGC (4) Absorber tube, glass tube, CPC without insulation, and FGC at the CPC aperture (5) Absorber tube, glass tube, CPC with insulation, and FGC at the CPC aperture. Compared to (1), U1 is reduced by 9-16% in (2), 14-31% in (3), 16-40% in (4), and 21-45% in (5). Configuration (5) showed the best performance with 10.8% lower U1 than (3). A numerical model using surface-to-surface radiation and realizable k-epsilon turbulence, together with an analytical model, predicted results within 6.6% of experiments.

The phenomenon of ablation occurring in sacrificial concrete surrounding the nuclear reactor is studied by conducting experiments with oxyacetylene welding. The oxidizing flame produced from the oxyacetylene welding is a potential source of high heat flux and temperature exposed to the ferrosiliceous concrete (contains hematite aggregates) and ordinary concrete (without hematite aggregates). The ablation caused by the highintensity oxidizing flame is observed to be highly non-uniform. The maximum temperature rise, ablation depths and overall mass loss in ordinary concrete are observed to be higher compared to the ferrosiliceous concrete. The presence of hematite in ferrosiliceous concrete has reduced the heat diffusion, ablation depth and mass of ablated material inside the concrete and hence exhibited better ablation characteristics. A semi-infinite heat transfer model ignoring the effects of chemical reactions are formulated and estimated the approximate interfacial heat flux profiles develop at the interface between the flame and concrete. The outcomes of the study conclude that the ferrosiliceous concrete can withstand the high heat fluxes of the oxidizing flame, and hence, it will withstand the adverse scenario of the interaction of molten corium with concrete walls occurs during the failure of nuclear reactors.

In the present study the appropriate reference temperature is identified for the compressible impinging jet. Using the measured reference temperature, local recovery factor is calculated. The steady state thin metal foil technique is used for the measurement of target plate temperature. In this study, the effect of Mach number (Ma) at a constant Reynolds number (Re) and the combined effect of Mach number and Reynolds number on the heat transfer rate are investigated. For both the cases, jet-to-plate distance is varied from z/d = 5 to 12. For the first case (effect of Mach number at a constant Re = 20,000), Ma is varied from 0.15 to 0.85. In the second case (combined effect of Mach number and Reynolds number), Ma is varied from 0.2 to 0.78 and the corresponding Re variation is 7200 to 29,000. At a constant Reynolds number, the heat transfer coefficient increases with the increase in the Mach number. For a given Mach number and Reynolds number, the heat transfer rate decreases with the increase in the jet-to-plate distance. The recovery factor remains unaffected by the Mach number and jet-to-plate distance in the case of the concurrent variation of the Mach number and Reynolds number.

Subcooled flow boiling of water is widely observed in high heat flux and high mass flux (HHHM) cooling applications such as heat exchangers, refrigeration equipment, boiler tubes and nuclear reactor core fuel channels in pressurized heavy water reactors (PHWR). In this study, the focus is on investigating the local heat transfer coefficient (HTC) and pressure drop in a horizontal tube experiencing subcooled boiling of water under low pressure and HHHM conditions. The study encompasses different geometrical parameters such as tube diameter (5.5 mm , 7.5 mm , 9.5 mm and 12 mm ) and length (550 mm for each of the tubes). The operating parameters that are varied include mass flux (248-2000 kg/m2.s) and heat flux (0-1837 kW/m2). Infrared thermography is used to measure the local wall temperature. A non-dimensional correlation for the diabatic pressure drop ratio (ratio of diabatic pressure drop to adiabatic pressure drop) as a function of Jakob number (Ja), Boiling number (Bo) and diameter ratio is developed. Subcooled boiling pressure drop ratio for 5.5 mm , 7.5 mm and 9.4 mm diameter tubes is 2.23 which is independent of diameter. A correlation for the two phase local HTC during subcooled flow boiling conditions as a function of Ja , Bo and Prandtl number (Pr) is also developed.

Heat transfer in a flat plate with metal foam under impinging jet conditions is a complex combination of conduction (finned) and convection (unfinned) heat transfer. This study reports an analytical approach for the quantification of finned and unfinned heat transfer from a targeted plate with metal foam under impinging jet conditions. Along with the quantification of heat transfer modes, the interstitial heat transfer, the efficiency of metal foam as a fin, and thermal resistances are also quantified analytically. The analysis is carried out for rectangular slot jet and multiple air jet impingement conditions. The varying parameters are jet-to-plate spacing, metal foam thickness, and Reynolds number. The results suggest that for the slot jet case, the finned and unfinned heat transfer is around 70 and 30 percent of the total heat transfer independent of the foam thickness. However, for multiple jet case, finned and unfinned heat transfer is around 50 percent each except for 12 mm thickness. The interstitial heat transfer coefficient and fin efficiency increase with a decrease in the thickness of the foam. For both slot and multiple jet impingement cases, the thermal resistance to unfinned heat transfer is greater in comparison with the finned heat transfer. The presence of metal foam on the flat plate incenses the overall heat transfer by two times the smooth flat plate.

This study investigates local heat transfer distribution and pressure drop in octet-structured aluminium foam (AlSi10Mg) within a rectangular channel. The local heat transfer distribution is analysed using a thin metal foil technique and an IR camera. The foam has a thickness of 12.5 mm and 70.6 % porosity. Experiments were performed to study the effect of Reynolds numbers on heat transfer performance and assess the contribution of conductive versus convective heat transfer. A range of Reynolds numbers were tested from 1000 to 25000. The heat transfer coefficient asymptotes to a constant value beyond the Reynolds number of 20000. To segregate the effect of fin and wall heat transfer, separate experiments are conducted using resin foam. The effect of conduction heat transfer (fin) dominates convection heat transfer (wall), with a contribution ratio of 81-19 %. Metal foam exhibits a 3.5-5 times higher heat transfer coefficient than resin foam. The performance enhancement over smooth channels is 5 times higher for resin foam and 18 to 29 times higher for metal foam. The Performance Evaluation Criterion for metal foam ranges from 2.02 to 3.11, while for resin foam, Performance Evaluation Criterion is between 0.56 and 0.63. The Constant Pumping Power Criterion shows a 32 % higher thermal performance than the Performance Evaluation Criterion.

The present study focuses on the measurement of the local heat transfer distribution of a smooth flat plate impinged by an array of free surface jets on a thin metal foil. Local Nusselt number distributions are measured for a fixed jet diameter (d = 3 mm). The jet arrays consist of perfectly round apertures arranged in a square pattern, with a uniform spacing of 4d between adjacent jets in both streamwise and spanwise directions. A wide range of Reynolds numbers varying from 1000 to 12,500 are covered in this study. The nozzle to plate spacing (z/d) is varied between 1 and 10. The effect of the Reynolds number and nozzle to plate spacing on the local and spanwise average Nusselt number distributions are studied. The local and average Nusselt numbers are found to be symmetric in streamwise and spanwise directions and they follow a periodic pattern. The local Nusselt number exhibits an increase with nozzle-to-plate spacing within the low Reynolds number range (Re = 1000-2500). However, for Reynolds numbers exceeding 2500, the influence of nozzle-to-plate spacing on Nusselt number distributions remains negligible up to a nozzle-to-plate distance (z/d) of 5. Beyond this point, there is a gradual decrease in the Nusselt number value. The Nusselt number value gradually decreases beyond z/d of 5. At a Reynolds number of 1500, the Nusselt number increases by 71% for z/d = 10 in comparison to z/d = 1. Empirical correlations for local and spanwise average Nusselt number are proposed which satisfactorily predict the local as well as spanwise average Nusselt number distributions.

Vortex generator can be used to enhance heat transfer as it is capable of providing better mixing. Vortex generator is easy to manufacture and install in heat transfer units without much additional cost. Present work is planned to investigate the effect of vortex generators on local heat transfer coefficient, critical heat flux (CHF) and pressure drop to ensure design and safety of heat transfer units with vortex generators. Experiments are carried out for three values of pitch 22, 32 and 41 mm and an angle of attack of 45 degrees of vortex generators with horizontal configuration in a smooth tube of diameter 11.9 mm, length 1,000 mm and 0.4 mm wall thickness with R-123 as the working fluid. The effect of mass flux (175-681 kg/m2s) and heat flux (7.9 - 221.7 kW/m2) are studied. Both horizontal and vertical-oriented configurations of the vortex generator are tested. Local wall temperature measurement is carried out with Infrared thermography. Comparing results with plain tube shows the heat transfer coefficient and CHF enhancement. Horizontal orientation (HO) of the vortex generator gives better results than the vertical orientation (VO). Heat transfer coefficient, CHF and pressure drop penalty increase with a decrease in the pitch of the vortex generator.

Catastrophic fire incidents in nuclear reactors can cause rapid ablation of the concrete during molten corium concrete interactions (MCCI). In this work, experimental and numerical analysis are carried out to evaluate the interfacial heat flux profiles between the concrete and molten metal. The granite based concrete which contained Ordinary Portland Cement (OPC) is used and it's interaction with the molten mixture of MnO and TiO2 are studied. The transient temperature distribution inside the concrete and ablation depths are measured after pouring the molten metal into the concrete cavity. It is observed that the maximum ablation occurred at the bottom of the cavity due to initial severe interaction of molten metal with the concrete. The ablation depths at the base and the side walls are 28 mm and 15 mm, respectively. Comparison with 1D heat transfer and semiinfinite model confirmed that heat transfer in concrete can be analysed using semi-infinite medium approach up to Fourier number of 0.45. An inverse heat transfer problem is solved using sequential function specification method and estimated the interfacial heat flux profiles from measured transient subsurface temperature distribution. Subsurface temperatures are measured using sheathed K-type thermocouples. Consistent with the experimental observations, higher heat fluxes are observed for thermocouples located at the bottom and lower heat flux are observed for thermocouples placed at the side walls and corners. The heat flux profiles rise from the beginning, reach to a peak value and then, gradually decreased with time. Heat flux values are observed to be in the range of 1-19 kW/m2. The experimental observations of this study are compared with the literature and the possible values of ablation depths for the case of the actual nuclear reactor are estimated.

The local heat transfers of a thermally developing region in a rectangular channel filled with porous metal foam are investigated experimentally. A thin metal foil technique and thermal IR imaging are adopted for the measurement of the local temperature distribution. An open-cell metal foam made from copper having a porosity of 0.96 is used. The pore density of the foam is 30 PPI (pores per inch). Fluid flow characteristics like permeability and foam drag coefficient are measured by conducting local pressure drop experiments. Additionally, by utilizing local pressure measurement data, the non-dimensional pressure coefficient is quantified. The non-dimensional pressure coefficient increases in the streamwise direction, irrespective of the Reynolds number, and remains almost constant in the spanwise direction for Reynolds numbers greater than 2000. The effect of the metal foam thickness on local heat transfer and pressure drop is investigated for 13 and 20 mm thick porous metal foam. It is compared with a smooth channel to quantify the heat transfer augmentation in a channel filled with metal foam. Compared to a smooth channel, the channel with 20 mm thick metal foam shows 14 to 25 times augmentation in the Nusselt number. Similarly, the channel with 13 mm foam shows 10 to 15 times augmentation in Nusselt number. The increasing trend of the local Nusselt number is observed in a streamwise direction. The different criteria are studied to understand the thermal performance evaluation. In the Constant Pumping Power Criterion (CPPC), enhancements ranged from 3.14 to 6.72 for a 20 mm thickness and 2.62 and 4.34 for a 13 mm foam thickness. Similarly, in the Performance Evaluation Criterion (PEC), enhancement ranges from 2.23 to 4.83 for a 20 mm foam thickness and 1.89 to 3.16 for a 13 mm foam thickness. A generalized correlation is suggested to describe the average Nusselt number considering the foam material, foam thickness, pore density, and porosity as parameters.

Applications requiring high heat transfer rates, such as cooling of high-density electrical equipment, cooling of gas turbine components, cooling of rocket launcher components, cryosurgery, etc., are frequently use impinging jets. Non-uniformity in the heat transmission from the impingement surface is the main drawback of jet impingement heat transfer. In order to achieve uniform heat transfer, the current study examines the presence of porous carbon foam on a targeted surface. Using a thin metal foil and infrared thermography, the local heat transfer distribution of a porous carbon foamed surface is determined. The findings of the porous carbon foamed surface are compared to the bare surface (smooth surface without foam) for local Nusselt number and uniformity in the heat transfer (coefficient of variance). The effects of Reynolds number, foam height, and the distance between the nozzle exit to the targeted plate are examined. The results of the carbon foamed surfaces are also compared with the aluminium metal foamed surface results available in the literature. The current work also describes the separation of the modes of heat transfer that exist with porous carbon foamed surfaces while under jet impingement. The findings imply that, depending on the height of the carbon foam, the porous carbon foam on a targeted surface gives a lower or equivalent heat transfer rate compared to a bare surface. In comparison to a bare surface, carbon foam on a targeted surface provides uniform heat transfer that is independent of foam height. The study of the separation of modes of heat transfer suggests that heat from the porous carbon foamed surface is conveyed by conduction induced by carbon foam and convection induced by jet fluid. The convection provided by the jet fluid is compromised by the carbon foam on a targeted surface. The conduction induced by carbon foam makes the heat transfer from the targeted surface more uniform. The conduction and convection factors can be used to present the conduction and convection heat transfer from porous carbon foamed surfaces, respectively. Regression analysis is used to develop a region-wise correlation for the conduction and convection components. The local Nusselt number of a carbon foamed flat plate can be predicted using the local Nusselt of a bare surface utilizing the provided correlations for conduction and convection factor.

A smooth flat plate impinged by multiple jets is investigated for the local heat transfer using a thermal imaging technique. An inline arrangement of circular jets with all side exit scheme is implemented. Jets are introduced from a 4 mm thick orifice plate. The jet diameter is 3 mm. Reynolds numbers (based on the jet diameter) range covered in this study are from 500 to 15,000. The variation of the local, spanwise average, and overall average Nusselt numbers is investigated for nozzle exit to targeted plate spacing of 0.5 d-6 d, where d is the jet diameter. A periodic pattern is witnessed in the local Nusselt number with a drop in its amplitude in the spanwise direction. For a given Reynolds number, the local and average Nusselt numbers are influenced by nozzle exit to targeted plate spacing. The influence of Reynolds number on the local and average Nusselt number is captured using Ren. The value of n is found to be 0.5 and 0.6 depending on the Reynolds number range. In comparison with a single jet, multiple jet impingement on a smooth flat plate deteriorates the local Nusselt number. A semi-empirical correlation that captures the periodic nature of local and average Nusselt numbers is suggested.

Heavy density concrete made of hematite-based aggregates commonly known as ferro-siliceous sacrificial concrete (FSSC) can be used effectively against radiation shielding in case of nuclear melt down. However, performance assessment of such concretes on exposure to elevated temperatures are not available widely. In the present study, thermo-mechanical and thermophysical properties of hematite-based FSSC concrete are evaluated on exposure to elevated temperatures. The porosity and water absorption capacity of FSSC are investigated on its exposure to higher temperatures ranging from 30 °C to 1000 °C. Similarly, various thermo-mechanical [compression, split tension, modulus of elasticity (MOE)] and thermophysical (specific heat, thermal conductivity, thermal diffusivity, and thermal effusivity) properties are evaluated on exposure to elevated temperatures. The mechanical properties of the concrete such as compression, split tension, MOE decrease monotonically with increase in the exposed temperature. Porosity and water absorption of the concrete increases with the increase in temperature. Relationship between damage and the exposed temperature can be described by Weibull model. Thermophysical property such as specific heat of the concrete increases, while thermal conductivity, thermal diffusivity and effusivity decrease with the increase in temperature. Correlations for the variation in thermal conductivity, specific heat and thermal diffusivity are proposed. The variations in the mechanical and thermophysical properties are explained through the porosity variations and correlation coefficient between porosity and various mechanical and thermophysical properties.

Several passive techniques can substantially improve the heat transfer performance of conventional heat exchangers. Passive methods persistently use heat transfer augmentation inside tubular sections, which tackle dominant thermal resistance. The thermal resistance in turbulent flow is primarily due to a thin viscous sublayer near the tube wall. This proliferates into the cross-section of the conduit in the case of laminar flows due to the existence of a relatively thicker boundary layer. This urges disturbance in the entire fluid throughout the cross-section in laminar flows, whereas augmentation devices are usually located close to the wall for turbulent flows. Most of these methods yield increased fluid residence time within the system by inducing swirling motion. The form of disturbance in the flow field is the characteristic of the passive technique used to prolong the fluid residence time and is exhibited differently in both flow regimes. The present article showcases a comprehensive review of heat transfer enhancement through thermo hydraulic performance assessment of these methods reported in the literature. The comparison is based on ratios of Nusselt numbers, first at the same Reynolds numbers, and then, at equal pumping power with constant heat transfer area.

An experimental investigation was conducted to examine critical heat flux (CHF), boiling pressure drop, and local heat transfer coefficient in a horizontal channel with transverse grooves for flow boiling with R-123. Measurement of the wall temperature of the channel was done using a thermal camera. Five test sections were used for the study. A combination of groove pitch (p) of 18.6 and 11 mm and groove height (e) of 1 mm and 0.55 mm were used for the study. The groove pitch-to-height (p/e) ratios used were 11, 20, 18.6, and 33.8. Experiments were carried out for different heat fluxes in the range from 180 to 1259 kW/m2, six different mass fluxes from 180 to 1259 kg/m2s, and Reynolds numbers from 5834 to 51770. In the grooved channel compared with the smooth tube, an improvement in heat transfer and CHF with the higher pressure drop was found. The boiling heat transfer coefficient increases with a decrease in (p/e) ratio. The enhancement of heat transfer coefficient and CHF improves with a decrease in (p/e) ratio. The maximum enhancement of CHF for (p/e) ratios of 11, 18.6, 20, and 33.8 is found to be 134%, 87%, 57%, and 53%, respectively. An abrupt increase in test channel wall temperature is an indication of CHF occurring. CHF was observed at the top portion of the grooved channel and was found to be of departure from nucleate boiling type.

Steam inside the containment should be condensed to reduce the system pressure after severe accidents such as loss of coolant accident (LOCA) and main steam line break (MSLB). Due to the absence of an active coolant system, the passive containment cooling system (PCCS) can be used to improve safety by condensing the steam on a test section (coolant) tube surface. However, the steam condensation is degraded by the presence of non-condensable gases like air. The condensation heat transfer coefficient is dependent on operating parameters such as system pressure, air-mass fraction, and wall subcooling and geometric parameters such as tube diameter, tube length, and angle of orientation. In the present study, experiments are conducted on a vertical tube to estimate the condensation heat transfer coefficient in the presence of air. The system pressure ranges from 2 - 5.5 bar, air-mass fraction of 0.3 - 0.8 and wall subcooling of 27 - 80 degrees C. The influence of test section tube diameter (48.2 mm, 32 mm, 22 mm, and 16 mm) and inclination angles (90 degrees and 68 degrees from the horizontal) are investigated. Based on the experimental data, a correlation for condensation heat transfer coefficient in terms of system pressure, air-mass fraction, wall subcooling, tube diameter and angle of inclination is proposed. The condensation heat transfer coefficient increases with the increase in the system pressure and decreases with the increase in the air mass fraction, wall subcooling and condenser tube diameter. With the decrease in the diameter of the vertical condenser tube from 48.2 mm to 16 mm, an increase in the condensation heat transfer coefficient of 66% is observed. A non-dimensional correlation for condensate Nusselt number as a function of Grashof number, air-mass fraction and Jakob number is also proposed. This correlation predicts 80% and 90% experimental data within 20% and 30% uncertainty range, respectively. (c) 2023 Elsevier Ltd. All rights reserved.

The present study describes the heat transfer characteristics of free surface single-phase circular liquid jet impinging on a thin smooth flat surface subjected to uniform heat flux. The experimental setup is devel-oped to measure the local temperature of a thin stainless steel metal foil using the infrared thermography technique. The effects of nozzle diameter ( d = 2.5 to 10.8 mm), nozzle to plate spacing (z/d = 2 to 10), and Reynolds number ( Re = 50 0 0 to 24,0 0 0) on the local Nusselt number ( Nu ) distributions are studied. The local Nu is maximum in the stagnation region, and it decreases in the downstream flow following the stagnation region. The nozzle with a larger diameter shows maximum Nu compared to other nozzles. For a given z/d of 2 and a Reynolds number of 15,0 0 0, a maximum increment of 67% in local Nu is observed for a 10.8 mm nozzle in comparison to a 2.5 mm nozzle. The local Nu in the viscous boundary layer is found to be a function of the Reynolds number, Prandtl number, dimensionless r/d, and Weber number. The Nu increases with an increase in Reynolds number, however, the effect of the nozzle to plate spacing is negligible. The off-stagnation peak is observed for the 10.8 mm nozzle in the region of 0 <= r/d <= 0.7. The existing correlations reported in the literature are not able to capture the measured local Nusselt number distribution. Therefore, in the present study, an effort has been made to propose semi-empirical correlations for local Nu in the stagnation and viscous boundary layer region which will incorporate the diameter effect of the nozzle. The proposed correlations are able to predict the local Nusselt number distribution within the maximum deviation of 20%. The comparison between the free surface and sub-merged jet has been made which suggests that the local Nu for the submerged jet is higher compared to the free surface jet.(c) 2023 Elsevier Ltd. All rights reserved.

The current study investigates the distribution of local heat transfer coefficient, measurement of pressure loss and CHF in a lobed horizontal channel for flow boiling. Lobed channels with different lobed depths of 2.3, 2.0 and 1.25 mm, each having three different channel lengths (400, 600, and 1000 mm), are used for the experiments. The working substance used is R-123 with different mass flux. The heat flux range used is from 14 to 375 kW/m(2). Pressure drop, critical heat flux, and local heat transfer coefficients of the lobed channel are compared with corresponding smooth horizontal tubes.

Present study purposes a simple methodology for the measurement of thermophysical properties of the high performance ferro siliceous sacrificial concrete.The study involve simple experiments of temperature measurement and data processing to estimate various thermophysical properties such as specific heat, thermal conductivity, thermal diffusivity, and effusivity.The combination of these properties is exceptional and crucially advantageous to determine the ablation rate during interaction of the molten metal with concrete.The study based on one-dimensional semi-infinite transient heat conduction model.The specific heat of the FSSC is found to be 827.5 J/kgK while the value of thermal conductivity lies in the range of 7.4-8.8W/mk.The average value of thermal diffusivity and thermal effusivity is found to be 2.47×10 -6 m 2 /s and 4033.77.18 Jm -2 K -1 s -1/2 , respectively.

For designing a multiple jet impingement cooling system, the thickness of jet plate (a passage through which jet comes out) is one of the important parameters. Considering the thermal point of view, the thickness of the plate should be chosen so that it would enhance the heat transfer rate. In this study, the effect of jet plate thickness on the local heat transfer coefficient on a flat plate with multiple impinging air jets is investigated. The thickness ratios (t/d) of 0.5, 1.0 and 2.67 are studied. Reynolds number is varied from 5000 to 20,000 and jet-to-plate distance (z/d) is varied from 1 to 5. Jet diameter (d) is chosen to be 6 mm. The jet-to-jet distance (p/d) is kept constant at 3.Steady-state thin metal foil technique, along with infrared thermography, is used to obtain the temperature distribution and thereby the heat transfer coefficient. Coefficient of discharge for each jet plate, central line velocity decay and pressure distribution on the target surface are measured. These measurements provided new insights to better understand the heat transfer results in this study. For lower jet-to-plate distances (t/d= 0.5 to 1), heat transfer decreases with the increase in the jet plate thickness ratio. However, heat transfer is not affected with the further increase in the jet plate thickness ratio i.e., t/d = 1 to 2.67. Therefore, smaller jet plate thickness ratio is suitable to obtained high heat transfer rate. This study presents a new correlation for the local Nusselt number distribution or different jet plate thickness.