Numerical Investigation of the Tube Layout Effects on the Heat Losses of Solar Cavity Receiver

Thermoregulation of the thorax allows honeybees (Apis mellifera) to maintain the flight muscle temperatures necessary to meet the power requirements for flight and to remain active outside the hive across a wide range of air temperatures (Ta). To determine the heat-exchange pathways through which flying honeybees achieve thermal stability, we measured body temperatures and rates of carbon dioxide production and water vapor loss between Ta values of 21 and 45 degrees C for honeybees flying in a respirometry chamber. Body temperatures were not significantly affected by continuous flight duration in the respirometer, indicating that flying bees were at thermal equilibrium. Thorax temperatures (Tth) during flight were relatively stable, with a slope of Tth on Ta of 0.39. Metabolic heat production, calculated from rates of carbon dioxide production, decreased linearly by 43 % as Ta rose from 21 to 45 degrees C. Evaporative heat loss increased nonlinearly by over sevenfold, with evaporation rising rapidly at Ta values above 33 degrees C. At Ta values above 43 degrees C, head temperature dropped below Ta by approximately 1–2 degrees C, indicating that substantial evaporation from the head was occurring at very high Ta values. The water flux of flying honeybees was positive at Ta values below 31 degrees C, but increasingly negative at higher Ta values. At all Ta values, flying honeybees experienced a net radiative heat loss. Since the honeybees were in thermal equilibrium, convective heat loss was calculated as the amount of heat necessary to balance metabolic heat gain against evaporative and radiative heat loss. Convective heat loss decreased strongly as Ta rose because of the decrease in the elevation of body temperature above Ta rather than the variation in the convection coefficient. In conclusion, variation in metabolic heat production is the dominant mechanism of maintaining thermal stability during flight between Ta values of 21 and 33 degrees C, but variations in metabolic heat production and evaporative heat loss are equally important to the prevention of overheating during flight at Ta values between 33 and 45 degrees C.

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Effects of Absorber Emissivity on Thermal Performance of a Solar Cavity Receiver

Advances in Condensed Matter Physics ◽

10.1155/2014/564639 ◽

2014 ◽

Vol 2014 ◽

pp. 1-10 ◽

Cited By ~ 7

Author(s):

Jiabin Fang ◽

Nan Tu ◽

Jinjia Wei

Keyword(s):

Heat Loss ◽

Thermal Performance ◽

Thermal Efficiency ◽

Radiative Heat ◽

Slight Effect ◽

Radiative Heat Loss ◽

Total Heat Loss ◽

Convective Heat Loss ◽

Performance Curves ◽

Variation Tendency

Solar cavity receiver is a key component to realize the light-heat conversion in tower-type solar power system. It usually has an aperture for concentrated sunlight coming in, and the heat loss is unavoidable because of this aperture. Generally, in order to improve the thermal efficiency, a layer of coating having high absorptivity for sunlight would be covered on the surface of the absorber tubes inside the cavity receiver. As a result, it is necessary to investigate the effects of the emissivity of absorber tubes on the thermal performance of the receiver. In the present work, the thermal performances of the receiver with different absorber emissivity were numerically simulated. The results showed that the thermal efficiency increases and the total heat loss decreases with increasing emissivity of absorber tubes. However, the thermal efficiency increases by only 1.6% when the emissivity of tubes varies from 0.2 to 0.8. Therefore, the change of absorber emissivity has slight effect on the thermal performance of the receiver. The reason for variation tendency of performance curves was also carefully analyzed. It was found that the temperature reduction of the cavity walls causes the decrease of the radiative heat loss and the convective heat loss.

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Numerical Simulation of Natural Convection in Solar Cavity Receivers

Journal of Solar Energy Engineering ◽

10.1115/1.4029106 ◽

2015 ◽

Vol 137 (3) ◽

Cited By ~ 12

Author(s):

James K. Yuan ◽

Clifford K. Ho ◽

Joshua M. Christian

Keyword(s):

Experimental Investigation ◽

Natural Convection ◽

Heat Loss ◽

Convective Heat ◽

Heat Losses ◽

Thermal Engineering ◽

Convective Heat Loss ◽

Software Packages ◽

Receiver Model ◽

Cubical Cavity

Cavity receivers used in solar power towers and dish concentrators may lose considerable energy by natural convection, which reduces the overall system efficiency. A validated numerical receiver model is desired to better understand convection processes and aid in heat loss minimization efforts. The purpose of this investigation was to evaluate heat loss predictions using the commercial computational fluid dynamics (CFD) software packages fluent 13.0 and solidworks flow simulation 2011 against experimentally measured heat losses for a heated cubical cavity receiver model (Kraabel, 1983, “An Experimental Investigation of the Natural Convection From a Side-Facing Cubical Cavity,” Proceedings of the ASME JSME Thermal Engineering Joint Conference, Vol. 1, pp. 299–306) and a cylindrical dish receiver model (Taumoefolau et al., 2004, “Experimental Investigation of Natural Convection Heat Loss From a Model Solar Concentrator Cavity Receiver,” ASME J. Sol. Energy Eng., 126(2), pp. 801–807). Simulated convective heat loss was underpredicted by 45% for the cubical cavity when experimental wall temperatures were implemented as isothermal boundary conditions and 32% when the experimental power was applied as a uniform heat flux from the cavity walls. Agreement between software packages was generally within 10%. Convective heat loss from the cylindrical dish receiver model was accurately predicted within experimental uncertainties by both simulation codes using both isothermal and constant heat flux wall boundary conditions except when the cavity was inclined at angles below 15 deg and above 75 deg, where losses were under- and overpredicted by fluent and solidworks, respectively. Comparison with empirical correlations for convective heat loss from heated cavities showed that correlations by Kraabel (1983, “An Experimental Investigation of the Natural Convection From a Side-Facing Cubical Cavity,” Proceedings of the ASME JSME Thermal Engineering Joint Conference, Vol. 1, pp. 299–306) and for individual heated flat plates oriented to the cavity geometry (Pitts and Sissom, 1998, Schaum's Outline of Heat Transfer, 2nd ed., McGraw Hill, New York, p. 227) predicted heat losses from the cubical cavity to within experimental uncertainties. Correlations by Clausing (1987, “Natural Convection From Isothermal Cubical Cavities With a Variety of Side-Facing Apertures,” ASME J. Heat Transfer, 109(2), pp. 407–412) and Paitoonsurikarn et al. (2011, “Numerical Investigation of Natural Convection Loss From Cavity Receivers in Solar Dish Applications,” ASME J. Sol. Energy Eng. 133(2), p. 021004) were able to do the same for the cylindrical dish receiver. No single correlation was valid for both experimental receivers. The effect of different turbulence and air-property models within fluent were also investigated and compared in this study. However, no model parameter was found to produce a change large enough to account for the deficient convective heat loss simulated for the cubical cavity receiver case.

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Numerical Simulation of Natural Convection in Solar Cavity Receivers

ASME 2012 6th International Conference on Energy Sustainability, Parts A and B ◽

10.1115/es2012-91064 ◽

2012 ◽

Author(s):

James K. Yuan ◽

Clifford K. Ho ◽

Joshua M. Christian

Keyword(s):

Heat Flux ◽

Natural Convection ◽

Heat Loss ◽

Convective Heat ◽

Heat Losses ◽

Flat Plates ◽

Convective Heat Loss ◽

Software Packages ◽

Receiver Model ◽

Cubical Cavity

Cavity receivers used in solar power towers and dish concentrators may lose considerable energy by natural convection, which reduces the overall system efficiency. A validated numerical receiver model is desired to better understand convection processes and aid in heat loss minimization efforts. The purpose of this investigation was to evaluate heat loss predictions using the commercial computational fluid dynamics software packages FLUENT 13.0 and SolidWorks Flow Simulation 2011 against experimentally measured heat losses for a heated cubical cavity model [1] and a cylindrical dish receiver model [2]. Agreement within 10% was found between software packages across most simulations. However, simulated convective heat loss was under predicted by 45% for the cubical cavity when experimental wall temperatures were implemented on cavity walls, and 32% when implementing the experimental heat flux from the cavity walls. Convective heat loss from the cylindrical dish receiver model was accurately predicted within experimental uncertainties by both simulation codes using both isothermal and constant heat flux wall boundary conditions except at inclination angles below 15° and above 75°, where losses were under- and over-predicted by FLUENT and SolidWorks, respectively. Comparison with empirical correlations for convective heat loss from heated cavities showed that correlations by Siebers and Kraabel [1] and for an assembly of heated flat plates oriented to the cavity geometry [3] predicted heat losses from the cubical cavity within experimental uncertainties, while correlations by Clausing [4] and Paitoonsurikarn et al. [8] were able to do the same for the cylindrical dish receiver. No single correlation was valid for both receiver models. Different turbulence and air-property models within FLUENT were also investigated and compared in this study.

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Numerical study of Flow patterns and performance of a coupled cavity-dish system under different focal lengths

Transactions of the Canadian Society for Mechanical Engineering ◽

10.1139/tcsme-2021-0110 ◽

2021 ◽

Author(s):

Muhammad Uzair ◽

Mubashir Ali Siddiqui ◽

Usman Allauddin

Keyword(s):

Heat Loss ◽

Convective Heat ◽

Numerical Study ◽

Focal Length ◽

Heat Losses ◽

Operating Conditions ◽

Convective Heat Loss ◽

Parabolic Dish ◽

And Performance ◽

Coupled Cavity

The effectiveness of the parabolic dish system (PDS) is greatly affected by the heat losses associated with high temperatures. The complexity of flow and temperature patterns in and around the cavity receiver makes it a challenging task to determine the convective heat loss from the cavity. Various studies have been carried out to determine the convection heat losses from isolated cavities of different shapes. In the presence of dish structure, the free stream wind may affect the stability of structure and the heat losses from the PDS. In this study, effect of focal length on the performance of the coupled cavity-dish system was analyzed using numerical simulations. The loading and the convective heat loss from the cavity were examined with three different cavity positions and different operating conditions in the presence of the dish. The results showed that the shallow dish experienced higher local air velocities near the cavity receiver than in the case of the deep dish. It was concluded that the heat loss is a stronger function of tilt angle rather than focal length, and in essence, the heat losses due to variation of this are negligible.

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Heat dissipation during hovering and forward flight in hummingbirds

Royal Society Open Science ◽

10.1098/rsos.150598 ◽

2015 ◽

Vol 2 (12) ◽

pp. 150598 ◽

Cited By ~ 6

Author(s):

Donald R. Powers ◽

Bret W. Tobalske ◽

J. Keaton Wilson ◽

H. Arthur Woods ◽

Keely R. Corder

Keyword(s):

Surface Temperature ◽

Heat Loss ◽

Convective Heat ◽

Heat Dissipation ◽

Skin Surface ◽

Flight Speed ◽

Radiative Heat Loss ◽

Convective Heat Loss ◽

Body Regions ◽

Heat Budgets

Flying animals generate large amounts of heat, which must be dissipated to avoid overheating. In birds, heat dissipation is complicated by feathers, which cover most body surfaces and retard heat loss. To understand how birds manage heat budgets during flight, it is critical to know how heat moves from the skin to the external environment. Hummingbirds are instructive because they fly at speeds from 0 to more than 12 m s −1 , during which they transit from radiative to convective heat loss. We used infrared thermography and particle image velocimetry to test the effects of flight speed on heat loss from specific body regions in flying calliope hummingbirds ( Selasphorus calliope ). We measured heat flux in a carcass with and without plumage to test the effectiveness of the insulation layer. In flying hummingbirds, the highest thermal gradients occurred in key heat dissipation areas (HDAs) around the eyes, axial region and feet. Eye and axial surface temperatures were 8°C or more above air temperature, and remained relatively constant across speeds suggesting physiological regulation of skin surface temperature. During hovering, birds dangled their feet, which enhanced radiative heat loss. In addition, during hovering, near-body induced airflows from the wings were low except around the feet (approx. 2.5 m s −1 ), which probably enhanced convective heat loss. Axial HDA and maximum surface temperature exhibited a shallow U-shaped pattern across speeds, revealing a localized relationship with power production in flight in the HDA closest to the primary flight muscles. We conclude that hummingbirds actively alter routes of heat dissipation as a function of flight speed.

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Radiative Heat Loss Estimation of Building Envelopes based on 3D Thermographic Models Utilizing Small Unmanned Aerial Systems (sUAS)

Energy and Buildings ◽

10.1016/j.enbuild.2021.110957 ◽

2021 ◽

pp. 110957

Author(s):

Mark Leggiero ◽

Bradley Andrew ◽

Ryan Elliott ◽

John Indergaard ◽

JB Sharma ◽

...

Keyword(s):

Heat Loss ◽

Radiative Heat ◽

Loss Estimation ◽

Unmanned Aerial Systems ◽

Radiative Heat Loss ◽

Building Envelopes ◽

Aerial Systems

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Effect of radiative heat loss on steady hypersonic flow past a blunt body

Journal of Fluid Mechanics ◽

10.1017/s0022112067000989 ◽

1967 ◽

Vol 29 (3) ◽

pp. 485-494 ◽

Cited By ~ 2

Author(s):

M. I. G. Bloor

Keyword(s):

Numerical Solution ◽

Heat Loss ◽

Blunt Body ◽

Radiative Heat ◽

Constant Density ◽

Axially Symmetric ◽

Radiative Heat Loss ◽

Stagnation Region ◽

Expansion Procedure ◽

Axis Of Symmetry

Using the grey gas approximation, the effect of radiative heat loss on axially symmetric flows is studied. Using an expansion procedure about the axis of symmetry, a numerical solution for the stagnation region is found taking the shock to be spherical. The results of this calculation are compared with the results of Lighthill's non-radiative constant density solution.

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