Convective Heat Loss Research Articles

Optimization of Inclination Angle of Cavity and Characteristics of Attached Fins on the Absorber for Performance Enhancement of Solar Collectors

This research investigation was undertaken in ANDUAT, Kumarganj, Ayodhya, Uttar Pradesh, India to study the numerical optimization of natural convection heat suppression in a solar flat plate collector with straight fins. Optimal characteristics of an array of thin fins attached on the absorber plat were obtained by Particle Swarm Optimization algorithm (PSOA). Free convection considered dominant in the cavity. Governing equations contained continuity; momentum and energy are discretized by finite volume method. The medium is considered incompressible, whose free convection is dominant and Boussinesq approximation is applied. A simplified model of real systems is applied with free convection. Free convection problem is solved by SIMPLER algorithm. Two confined cavities with aspect ratios 30 and 60 are considered as flat plate solar collectors. The results indicate that significant reduction on the free convection heat loss can be obtained from solar flat plate collector by using plate fins, and an optimal plate fins configuration exit for minimal natural convection heat loss for a given range of Rayleigh number. Reduction of up to a maximum of 25% at 0 inclination angle was observed in aspect ratio 30. Results showed PSOA is able to obtain characteristics of attached adiabatic fins on the absorber plate also it can obtain optimal inclination angle of cavity to decrease heat losses from solar collectors. The results obtained provide a novel approach for improving design of flat plate solar collectors for optimal performance.

Prediction of Transient Temperature Distributions for Laser Welding of Dissimilar Metals

Distribution of temperature during the welding process is essential for predicting and realizing some important welding features such as microstructure of the welds, heat-affected zone (HAZ), residual stresses, and their effects. In this paper, a numerical model was developed using COMSOL Multiphysics of dissimilar laser welding (butt joint) of AISI 316L and Ti6Al4V thin sheet of 2.5 mm thickness. A continuous mode (CW) fiber laser heat source of 300 W laser power was used for the present study. A time-dependent prediction of temperature distributions was attempted. The heat source was assumed as a Hermit–Gaussian analytical function with a moving velocity of 120 mm/min. Both convective and radiant heat loss and phase change of the materials were considered for the analysis. In addition, variation of temperature-dependent material properties was also considered. The maximum and minimum temperature for the two materials at different times and the temperature in the different penetration depths were also predicted. It was found that the average temperature that can be achieved in the bottom-most surface near the weld line was more than 2400 K, which justifies the penetration. Averages of maximum temperatures on the weld line at different times at the laser spot irradiation were identified near 3000 K.The temperature fluctuation near the weld line was minimal and decreased more in the traverse direction. Scanning with a displaced laser relative to the interface toward the Ti6Al4V side reduces the maximum temperature at the interface and the HAZ of the 316L side. All of these predictions agree well with the experimental results reported in current literature studies.

Drying kinetics, heat quantities, and physiochemical characteristics of strawberry puree by Refractance Window drying system

AbstractAttempts were made to understand the drying kinetics, different heats involved, and change in physiochemical properties during Refractance Window (RW) drying of strawberry puree on an in‐house fabricated RW drier. RW drying of puree was conducted at 75°C water temperature with 1.5 mm thickness. The drying was carried out from initial moisture content 614% (db) to 5.1% (db) with1.2 m/s exhaust air velocity. RW drying took place only in the falling rate period. The sensible heat of strawberry puree, convective heat loss from puree surface to air, and convective heat loss from the bottom surface (hot water to air) of the dryer were 70, 205, and 61.96 W, respectively. The evaporative capacity of 5.28 kg m−2 hr−1 was calculated at experimental parameters. This confirms that the drying system is advantageous. RW dried strawberry puree was rehydrated as exact of fresh strawberry puree considering total solids. Total phenolic content and ascorbic acid were significantly (p < .05) reduced from 246.33 mg gallic acid equivalent (GAE) (fresh strawberry puree) to 204.67 mg GAE per 100 g (reconstituted RW dried strawberry puree); 33.31–28.54 mg/100 g, respectively. Color characterization through L*, a*, b*, darkness factor (b*/a*), chroma (C), and hue angle (h*) revealed that reconstituted RW dried puree sample was darker compared with fresh puree.Practical ApplicationsAmong various drying methods of fruit purees, hot air drying is the most widely used. It results in an unpleasurable taste and color. During the falling rate drying period, the less thermal conductivity of fruit puree restricts heat transfer in conventional drying. Refractance Window (RW) drying adopts low‐temperature drying for less time, which leads to advantages like effective and economical drying. RW drying technology can be promising at the commercial level in product quality and much‐reduced production cost.

Investigation of heat transfer characteristics of hydrogen combustion process in cylindrical combustors from microscale to mesoscale

In the field of micro and mesoscale combustion, the feature of flame-wall thermal coupling is of great significance because of its small scale nature. Thus, this work provides a comprehensive heat transfer analysis in cylindrical combustors from the perspective of numerical simulation. The combustor has a fixed length-to-diameter aspect ratio of 10, and the channel diameter is scaling up from 1 mm to 11 mm to explore the influence of chamber dimension on heat transfer and flame structure. The distribution of convective and radiative heat flux on inner surface, contribution of thermal radiation are given. Moreover, the role of radiation in flame structure is analyzed, and the convective and radiative heat losses are quantitatively analyzed. We find that radiative heat flux is smaller compared to convective heat flux, and the proportion of radiative heat flux becomes larger with an increasing diameter. Thermal radiation does not change the flame structure when the diameter is less than 3 mm. When the diameter is greater than 5 mm, thermal radiation changes the location of flame front. The heat loss becomes larger at a smaller diameter, and heat loss ratio can reach approximately 73.6% in the combustor with diameter of 1 mm.

Thermal explosion of a reactive gas mixture at constant pressure for non-uniform and uniform temperature systems

In this study, the approximate and exact solutions for the stationary-state of the solids model with neglecting reactant consumption for both non-uniform and uniform temperature systems were applied on gas ignition under a constant pressure condition. The criticality conditions for a slab, an infinite cylinder, and a sphere are determined and discussed using dimensionless temperatures under constant ambient and surface temperatures for a non-uniform temperature system. Exact solution for a Semenov model with convection heat loss was also presented. The solution of the Semenov problem for constant volume or density as a solid and constant pressure were compared. The critical parameter δ is calculated and compared with those of Frank-Kamenetskii solution values. The validation of the calculated ignition temperatures with other exact solution and experimental results were offered. The relation between critical parameters form Semenov and F.K. models solution was introduced.

Study on Spectral Radiative Heat Transfer Characteristics of a Windowed Receiver with Particle Curtain

In this paper, a windowed receiver with a particle curtain is numerically simulated under full-spectrum conditions. The discrete phase model (DPM) is used to model the particle flow and interactions between the particle phase and the air phase. The scattering, absorption of the particle curtain and quartz glass window are considered in detail. The spectral characteristics of glass have an important influence on the heat transfer characteristics and the receiver efficiency. The results show that the quartz window can reduce the convective heat loss and the cavity re-radiation heat loss. Under the same conditions, the receiver efficiency of a windowed receiver with a particle curtain is increased by 11.9% compared with an aerowindow receiver with a particle curtain. Under the same mass flow, the particle curtain thickness and particle size have a non-negligible influence on the flow pattern and temperature distribution of the particle curtain. When the particle curtain thickness is low, the flow stability of the particle curtain is high; as the particle curtain thickness increases, the volume fraction of the particle curtain decreases, and the flow stability of the particle curtain decreases, which affects the shape of the curtain. The scattering and absorption characteristics of the particles are different, resulting in different net fluxes of incident radiation under the reflection of the particle curtain and the back wall. As the particle curtain thickness increases, the particle average exit temperature and the receiver efficiency show a trend of first increasing and then decreasing. When d = 30 mm, the incident radiation (G) at the position of the particle curtain is larger, the particle average exit temperature reaches 1156.72 K, and the receiver efficiency reaches 74.4%. Therefore, different particle sizes also have a significant impact on the flow pattern of the particle curtain and the radiation distribution inside it. In the range of 250–750 μm particle size, the particles average exit temperature reaches above 1150 K, and the receiver efficiency is above 72.6%. As the particle size increases, the particle average exit temperature, and the receiver efficiency show a trend of first decreasing and then increasing. When the particle size is 500 mm, the particle average exit temperature reaches 1175.8 K, and the receiver efficiency reaches 79.4%.

An air curtain surrounding the solar tower receiver for effective reduction of convective heat loss

External-type solar tower receiver, mounted on the top of a tower with an elevation of dozens or even hundreds of meters, is faced with unavoidably massive convective heat loss due to high operating temperature and high-speed wind at high altitude. To effectively reduce the convective heat loss, a novel solar tower receiver with an air curtain is proposed and designed. In this study, the comprehensive thermal performance of plant-scale tower receivers is numerically studied based on the finite volume method. The impacts of different configurations of air nozzles on the thermal performance of the solar tower receiver are investigated. Furthermore, techno-economic analyses on the Solar Two power plant are also carried out. The results show that, in scenario of incoming wind speed of 15 m/s, the air curtain exerts the best roles at the air-jet angle of 45°, a distance of 1.50 m from the receiver surface, and air-jet velocity of 16 m/s in an operating mode of switching off the air nozzles at the windward and leeward sides. Compared with the original receiver, the heat loss of the proposed receiver is effectively reduced by 9.60 %, and the efficiency and electricity power production are enhanced by approximately 0.49 %.

Investigation of Microclimate Ventilation of Simulated Garment in Terms of Wind Speed and Air Gap Thickness

During the circulation of air in the garment’s microclimate, the body extra heat dissipates to the environment. This event is an important factor in thermal comfort and ventilation. In this study, a heating cylinder was designed and calibrated to assess the ventilation occurrences in garment as a simulator of the human body. Tubular samples with different sizes were prepared using double jersey weft knitted fabric in order to obtain five levels of air gap thickness, including −1.5, 0, 1, 1.5 and 2 cm. Experiments were carried out in two conditions including without wind and by wind blowing with eight different wind speeds include 0.5, 1, 1.5, 2, 2.5, 3, 5 and 7 m/s. In each condition, the air gap’s temperature was considered as the main parameter for examination of the microclimate ventilation. The obtained outcomes revealed that by increasing the air gap thickness to more than 1 centimetre, the ventilation rate increased significantly. In the absence of wind, the change in the air gap’s temperature was not considerable for all sizes due to the natural convection phenomena that were slow. An increase of wind speed to more than 1 m/s led to a rise in the forced convection heat loss and took less time for the system to reach the equilibrium temperature; hence, higher ventilation rate was achieved. Moreover, by the increment of wind speed, the reduction of microclimate temperature follows a power function and at higher wind speeds remained approximately constant. According to the statistical analysis of results, it was confirmed that the ventilation rate of garment was influenced by wind speed; however, the influence of air gap thickness only in the absence of wind is significant.

Combined convective loss from a bicylindrical cavity receiver under wind condition: an experimental study

ABSTRACT The solar cavity receiver performance is considerably influenced by wind specifications in the parabolic dish system. This paper performs experiments to study the effects of wind direction and speed on the heat losses from a bicylindrical cavity receiver at different inclinations. A range of wind speeds from 0.3 to 5.7 m/s and the three head-on, back-on, and side-on wind directions for the side-facing and downward-facing cavity situations are examined. The cavity surface temperature distribution shows a jump due to the particular shape of the cavity and the sharp corner between two cylinders. The results show that the head-on wind yields a higher combined convection heat loss in contrast to the back-on and side-on wind directions for all wind speeds and cavity inclinations except 90° (vertically downward-facing) where the effects of wind directions are the same. For the lower side-on wind speeds, increasing cavity inclination results in decreasing combined convective loss, while on the contrary the change in the combined convective loss is very small regardless of the cavity inclination at higher wind speeds above 4 m/s. Moreover, for the back-on wind, comparing to the other wind directions, the combined convective loss is lower. For all wind directions and speeds, the horizontal cavity receiver gives lower forced convective loss compared with the natural convective loss. For the downward-facing cavity, an empirical correlation has been developed for the combined Nusselt number with respect to the Reynolds number, Grashof number, cavity inclination, wind direction, and the absolute temperature ratio. The proposed correlation supports the experimental results by a deviation within ±20%. Moreover, the uncertainty analysis revealed that the relative expanded uncertainty of combined Nusselt number varies between 2.1% and 16.3%, depending upon the cavity inclination, wind speed and direction.

Experimental and numerical analysis of anti-icing a GLARE laminate

Abstract This study aimed at developing a numerical model to estimate the power consumption of a GLAss fiber REinforced aluminum-based electro-thermal ice protection system for anti-icing. The GLAss fiber REinforced aluminum laminate was embedded with recurred S-shaped copper meshes thermally energized by a surface power density. The model determined the influence of a forced convective heat loss and copper-mesh dimension on anti-icing. As a test case, the numerical model addressed the anti-icing performance of a GLAss fiber REinforced aluminum 5A-3/2 laminate at a sub-freezing ambient temperature of 267.95 K. The model demanded a surface power density of 13.03 kW/m2 in order to keep the temperature of the outer aluminum-skin at 285.44 K, which was clearly beyond the water freezing point. The numerically predicted power consumption for anti-icing was in excellent agreement with that from the experiment. The experiment was conducted in a miniature icing wind tunnel by incorporating the comparable icing condition of the numerical analysis. In the experiment, an infrared thermography camera determined the spatio-temporal temperature history of the outer aluminum skin in contact with the free-stream. The infrared thermography camera measurements of surface temperature collapsed in close proximity to that from the numerical analysis. The outcomes of this study will consolidate the confidence of aircraft manufacturers on the proposed anti-icing system.

Numerical modeling of non-premixed combustion of micron-sized Ti particles in counter-flow arrangement: The effects of heat losses

In this study, a numerical model was introduced for simulation of preparation of TiO2 by non-premixed combustion of Ti particles in counter-flow arrangement. Preheating, reaction, melting, and vaporization processes were modeled using the asymptotic concept. To reach a viable model, convective and radiative heat losses were included in the model derivation. The conservation equations for mass and energy were presented in each zone with the heat loss terms. Boundary and jump conditions were employed to solve the governing equations to preserve continuity throughout the system. The numerical modeling was first validated against literature data, where good agreement was achieved. Then, the focus of results and discussion were given to the influence of heat loss terms on the spatial distributions of temperature and species mass fractions. It was clearly shown in that heat losses can significantly shorten the distances between the stagnation plane and flame fronts. This is the also the case for melting and vaporization fronts. However, the heat losses can obviously suppress thermophoretic force. Finally, the effects of initial fuel temperature on positions of flame, melting, and vaporization fronts, where almost linear dependences were found that higher initial fuel temperature results in closer fronts to the stagnation plane.

Convective and Radiative Heat Losses of a Cylindrical Accumulator for Solar Water Heating Collectors

Convective and Radiative Heat Losses of a Cylindrical Accumulator for Solar Water Heating Collectors

Acoustic parametric instability, its suppression and a beating instability in a mesoscale combustion tube

The present work reports thermo-acoustic instability in a mesoscale tube of diameter 8 mm (> quenching diameter) and length 702 mm. Mixtures with Lewis number, Le~ 0.8 (rich C2H4/O2/CO2), 1.05 (lean C2H4/O2/CO2) and 1.34 (lean C2H4/O2/N2) are used. Several new flame responses are observed. For lower burning velocity mixtures, flame extinction is observed due to heat loss when primary instability transforms to secondary instability for all Le mixtures. However, parametric cellular structures which are characteristic of parametric instability are observed only for Le of 0.8. It is proposed based on calculations and experiments that parametric structures will be observed only when diameter of tube is two times larger than the characteristic wavelength of parametric instability. If the tube diameter doesn't allow formation of parametric structure whirling and counter-rotating flames are observed instead of parametric structures. Suppression of acoustic parametric instability is observed for higher burning velocity mixtures with Le>1 and its mechanism is discussed. For a range of SL, a beating instability is observed for CO2 diluted mixtures of Le>1, where pressure oscillation and flame motion show beating oscillations of frequency around 15 Hz. This beating instability is believed to be caused by non-linear interaction of acoustic instability with pulsating instability of flame front which is caused due to combined radiative and convective heat loss. Due to CO2 dilution, the radiative heat losses could play a significant role in inducing pulsating instability.

Field measurement of the urban pedestrian level wind turbulence

The utilization of outdoor spaces is affected by its thermal environment, and wind, which determines the convective heat loss from the human body, is one of the most influencing factors of thermal comfort. There is a lack of study of the correlation between the convective heat loss and the wind turbulence at the pedestrian height. In this study, we measured the wind speed and turbulence characteristics within the first 2-m above ground level at three city green-spaces surrounded by buildings of different heights and densities. To accurately calculate the turbulence intensity from the time-series of wind velocity data, the synoptic trend was removed, otherwise it may cause over-estimation of the 10-min average turbulence intensity by 30–60%. The present onsite wind measurements show that the average turbulence intensity ranges from 22 to 48%, and the turbulence scale is within 1.2–12.0 m. Since wind effect on convective heat transfer is mediated through its impact on the convective heat transfer coefficient, which is often obtained experimentally in a controlled environment, the onsite wind data from this study provides a reference for the design of the experimental conditions.

All-Cold Evaporation under One Sun with Zero Energy Loss by Using a Heatsink Inspired Solar Evaporator.

Interfacial solar steam generation is a highly efficient and sustainable technology for clean water production and wastewater treatment. Although great progress has been achieved in improving evaporation rate and energy efficiency, it's still challenging to fully eliminate the energy loss to the surrounding environment during solar steam generation. To achieve this, a novel heatsink‐like evaporator (HSE) is developed herein. During solar evaporation, the temperature on the top solar evaporation surface can be regulated by the fin structures of the HSE. For the evaporators with 5 to 7 heatsink fins, the temperature of the solar evaporation surface is decreased to be lower than the ambient temperature, which fully eliminates the radiation, convection, and conduction heat losses, leading to the absolute cold evaporation over the entire evaporator under 1.0 sun irradiation. As a result, massive energy (4.26 W), which is over 170% of the received light energy, is harvested from the environment due to the temperature deficit, significantly enhancing the energy efficiency of solar steam generation. An extremely high evaporation rate of 4.10 kg m−2 h−1 is realized with a 6‐fin photothermal HSE, corresponding to an energy conversion efficiency far beyond the theoretical limit, assuming 100% light‐to‐vapor energy conversion.

Enhancing the resolution of the angular orientations of the flow controlling blades on a sustainable house by training an artificial neural network

Among all its effects, the development of the boundary layer, its separation, and formation of the wake region could lead to higher convective heat transfer over the body, if the flow conditions cause high gradient velocity profiles in the surface vicinities of the field. And also, a low-pressure region in the downstream of the geometry is formed, which increases the pressure drag exerted on it. The influence of the aforementioned issue on the zero energy house design has been tackled by introducing a new flow control mechanism. The so-called flow controlling blades (FCBs) were recently designed and investigated on a smart sustainable house, in order to control the flow field around the house, prevent the separation, and decrease the wake intensity, targeting a lower level of convective heat loss and drag force exerted on the body. The angular orientation of these FCBs was formerly determined for 12 different free wind directions (30° increments), as a look-up table for the main control system of the house. To increase the resolution of the orientations, we make use of a recently successful tool in machine learning called neural networks to estimate the desired orientation of the blades for the wind directions that do not exist in the said look-up table. Consequently, all the sample investigated sub-intervals not originally covered by the CFD data, showing great coincidence with the data driven from the neural network utilized in this study.

Metal foam/PCM melting evolution analysis: Orientation and morphology effects

Phase Change Materials (PCM) are promising materials for thermal energy storage systems. Since they present a relatively low thermal conductivity, they are often embedded in an open cell a metallic foam to enhance the overall thermal conductivity. In this paper, both experimental and numerical results on PCMs coupled with aluminum foams under different heat fluxes, porosities, number of Pores Per Inch (PPIs) and orientation are presented. The test cell is equipped with a Zincum Selenide window that allows to capture the whole temperature distribution by means of a IR camera. The melting front position in time is tracked by means of a MATLAB® algorithm based on IR camera images that are useful for a more robust tracking of melting front. Numerical simulations are performed with references to the porous media volume-averaged approach, under the assumption of local thermal non-equilibrium between the two phases. The most updated correlations for the porous media closing coefficients are taken from the literature. All the experiments are compared with numerical simulations, showing a very good agreement. After showing the effects of the different input parameters on melting front evolution, an analysis in terms of different convective heat losses to the environment and melting temperature range is presented to appreciate how these two variables affect the melting front position. Finally, total melting front evolution has been compared between experiments and simulations, showing a good agreement. This has been evaluated for different conditions, showing that a decrease in the porosity drastically reduces the melting time, while PPI has no relevant effect and small effects can be observed from orientation.

Estimation of convective heat losses from conical cavity receiver of solar parabolic dish collector under wind conditions and receiver orientations

The parabolic dish collector is one of the recognized concentrated solar power systems based on point focusing, which provides high-temperature heat, high concentration ratio, and low heat loss. This system consists of a parabolic reflector and a cavity receiver situated in the focus line. In this work, the conical cavity receiver with an aperture diameter of 0.5 m is considered for a 100 m2 parabolic reflector having a focal to diameter ratio of 0.48. Due to the complexity of flow and temperature profile, the estimation of convective heat loss is a difficult task in a cavity receiver. More heat losses are associated with high temperature obtained in the cavity receiver of the parabolic dish collector. Due to diverse wind effect, the convective heat losses ramp up, which significantly influences the thermal performance of the concentrating power system. The present work aims to investigate the heat losses due to convection from the conical cavity receiver. The numerical investigation was performed using ANSYS Fluent 20R1 to calculate the convective heat losses from the conical cavity receiver of varying diameter to height ratio for varying wind speed, receiver orientation in head-on, and back-on wind flow directions. The considered influential parameters are varying from 0.5 to 1.5 for diameter to height ratio (d/h), 0° to 90° for receiver orientation (γ), 0 to 10 m/s for wind speed (V). The heat losses are highest at 60° and 75° receiver orientation for d/h = 0.5 and d/h = 1-1.5, respectively, at high wind speed in head-on condition, whereas in back-on wind condition, 30° receiver orientation has more heat losses among all the d/h values at high wind speed. The heat loss at 90° receiver orientation is low for 4-10 m/s. The trends of heat loss curve at receiver orientations for given wind conditions are similar for velocity more than 2 m/s. The result reveals that the considered influential parameters have a remarkable effect on convective heat losses from the cavity receiver.

Experimental investigation and heat loss analysis of a three-coil solar cavity receiver of parabolic dish collector under wind condition

A modified three-coil solar cavity receiver is experimentally investigated to observe the effect of increases in cavity inner wall area, fluid inlet temperature (50°C-80°C) and cavity inclination (θ = 0 to 90°) on the total heat loss from receiver under no wind and wind condition. The hot water is supplied from the top of the receiver and total heat loss is estimated experimentally. The total heat loss is reduced up to 1.2723-1.91 times (at θ = 0°-90°) as compared to single coil receiver at wind velocity 3 m/s. At wind velocity 3 m/s-5 m/s, the total heat loss increases up to 1.238-3.41 times at θ = 0° and 1.53-1.68 times at θ = 90°. The developed analytical model, estimates 1.52%-3.23% conduction, 1.36%-2.64% radiation and 97.11%-94% convection heat loss. With increase in cavity inclination the convection heat loss decreases sharply, while the radiation and conduction heat losses increase slowly.

Alternative Method to the Replication of Wind Effects into the Buildings Thermal Simulation

To design energy-efficient buildings, energy assessment programs need to be developed for determining the inside air temperature, so that thermal comfort of the occupant can be sustained. The internal temperatures could be calculated through computational fluid dynamics (CFD) analysis; however, miniscule time steps (seconds and milliseconds) are used by a long-term simulation (i.e., weeks, months) that require excessive time for computing wind effects results even for high-performance personal computers. This paper examines a new method, wherein the wind effect surrounding the buildings is integrated with the external air temperature to facilitate wind simulation in building analysis over long periods. This was done with the help of an equivalent temperature (known as Tnatural), where the convection heat loss is produced in an equal capacity by this air temperature and by the built-in wind effects. Subsequently, this new external air temperature Tnatural can be used to calculate the internal air temperature. Upon inclusion of wind effects, above 90% of the results were found to be within 0–3 °C of the perceived temperatures compared to the real data (99% for insulated cavity brick (InsCB), 91% for cavity brick (CB), 93% for insulated reverse brick veneer (InsRBV) and 94% for insulated brick veneer (InsBV) modules). However, a decline of 83–88% was observed in the results after ignoring the wind effects. Hence, the presence of wind effects holds greater importance in correct simulation of the thermal performance of the modules. Moreover, the simulation time will expectedly reduce to below 1% of the original simulation time.