Effect of Attic Ventilation on the Performance of Radiant Barriers

1992 ◽  
Vol 114 (4) ◽  
pp. 234-239 ◽  
Author(s):  
Mario A. Medina ◽  
Dennis L. O’Neal ◽  
W. Dan Turner
Keyword(s):  
Heat Flux ◽  
Flow Rate ◽  
Heat Fluxes ◽  
Cooling Load ◽  
Percentage Reduction ◽  
Percent Reduction ◽  
Resistance Level ◽  
Space Cooling ◽  
Percent Difference ◽  
Weather Variations

The objective of the experiments was to quantify how attic ventilation would affect the performance of a radiant barrier. Ceiling heat flux and space cooling load were both measured. Results of side-by-side radiant barrier experiments using two identical 13.38 m2 (nominal) test houses are presented. The test houses responded similarly to weather variations. Indoor temperatures of the test houses were controlled to within 0.2°C. Ceiling heat fluxes and space cooling load were within a 2.5 percent difference between both test houses. The results showed that a critical attic ventilation flow rate of 1.3 (l/sec)/m2 of the attic floor existed after which the percentage reduction in ceiling heat fluxes produced by the radiant barriers did not change with increasing attic airflow rates. The ceiling heat flux reductions produced by the radiant barriers were between 25 and 35 percent, with 28 percent being the percent reduction observed most often in the presence of attic ventilation. The space-cooling load reductions observed were between two to four percent. All results compiled in this paper were for attics with unfaced fiberglass insulation with a resistance level of 3.35 m2 K/W (nominal) and for a perforated radiant barrier with low emissivities (less than 0.05) on both sides.

scholarly journals A Study of Critical Heat Flux During Flow Boiling in Microchannel Heat Sinks

2011 ◽  
Vol 134 (1) ◽  
Author(s):  
Tailian Chen ◽  
Suresh V. Garimella
Keyword(s):  
Heat Flux ◽  
Critical Heat Flux ◽  
Flow Rate ◽  
Heat Input ◽  
Heat Sink ◽  
Flow Boiling ◽  
Strong Dependence ◽  
Heat Fluxes ◽  
Abrupt Decrease ◽  
Parallel Microchannels

The cooling capacity of two-phase transport in microchannels is limited by the occurrence of critical heat flux (CHF). Due to the nature of the phenomenon, it is challenging to obtain reliable CHF data without causing damage to the device under test. In this work, the critical heat fluxes for flow boiling of FC-77 in a silicon thermal test die containing 60 parallel microchannels were measured at five total flow rates through the microchannels in the range of 20–80 ml/min. CHF is caused by dryout at the wall near the exit of the microchannels, which in turn is attributed to the flow reversal upstream of the microchannels. The bubbles pushed back into the inlet plenum agglomerate; the resulting flow blockage is a likely cause for the occurrence of CHF which is marked by an abrupt increase in wall temperature near the exit and an abrupt decrease in pressure drop across the microchannels. A database of 49 data points obtained from five experiments in four independent studies with water, R-113, and FC-77 as coolants was compiled and analyzed. It is found that the CHF has a strong dependence on the coolant, the flow rate, and the area upon which the heat flux definition is based. However, at a given flow rate, the critical heat input (total heat transfer rate to the coolant when CHF occurs) depends only on the coolant and has minimal dependence on the details of the microchannel heat sink (channel size, number of channels, substrate material, and base area). The critical heat input for flow boiling in multiple parallel microchannels follows a well-defined trend with the product of mass flow rate and latent heat of vaporization. A power-law correlation is proposed which offers a simple, yet accurate method for predicting the CHF. The thermodynamic exit quality at CHF is also analyzed and discussed to provide insights into the CHF phenomenon in a heat sink containing multiple parallel microchannels.

scholarly journals Effects of External Heat Flux and Exhaust Flow Rate on CO and Soot Yields of Acrylic in a Cone Calorimeter

2021 ◽  
Vol 11 (13) ◽  
pp. 5942
Author(s):  
Sun-Yeo Mun ◽  
Jae-Ho Cho ◽  
Cheol-Hong Hwang
Keyword(s):  
Heat Flux ◽  
Mass Flow ◽  
Flow Rate ◽  
Cone Calorimeter ◽  
Mass Loss Rate ◽  
External Heat ◽  
Heat Fluxes ◽  
Temperature Fields ◽  
The Mean ◽  
External Heat Flux

The effects of changes in irradiance level (external heat flux), exhaust flow rate, and hood height on CO and soot yield were examined using a cone calorimeter. Black acrylic, having similar constituents as polymethyl methacrylate, was used as a combustible, and external heat fluxes ranging from 15 to 65 kW/m2 were considered. Both auto and spark ignitions were applied as ignition methods. The difference in auto and spark ignition methods had no effect on CO and soot yields, or on the mass loss rate (MLR), heat release rate (HRR), and effective heat of combustion (EHC), which are global parameters of fire. As the external heat flux increased, the mean MLR and HRR linearly increased while the EHC remained constant. When the external heat flux increased, the mean mass flow rates of CO and CO2 had a directly proportional relationship with the mean MLR. Consequently, CO and CO2 yields remained constant regardless of the external heat flux. In contrast, the mean mass flow rate and mean MLR of soot were linearly proportional as opposed to directly proportional, and the soot yield thus increased linearly with external heat flux. Variations in the exhaust flow rate and hood height, which can alter the velocity and temperature fields in post-flame and plume regions, had almost no impact on CO and soot yields, as well as on MLR and HRR. The results of this study are expected to provide improved insight into conventional approaches on the recognition of CO and soot yields as unique properties of each combustible.

Experimental Study on Two-Phase Heat Transfer of FC-72 in Microchannels Heat Sink

2009 ◽  
Author(s):  
Xiao Hu ◽  
Guiping Lin ◽  
Hongxing Zhang
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Pressure Drop ◽  
Flow Rate ◽  
Mass Flux ◽  
Heat Sink ◽  
Heat Fluxes ◽  
High Mass ◽  
Two Phase ◽  
Two Phase Heat Transfer

A closed-loop two-phase microchannels cooling system using a micro-gear pump was built in this paper. The microchannels heat sink was made of oxygen-free copper, and 14 parallel microchannels with the dimension of 0.8mm(W)×1.5mm(D)×20mm(L) were formed by electric spark drilling followed by linear cutting which separated the channels from each other. The heat transfer performance was evaluated by the fluid temperature, the pressure drop across the micro-channels and the volumetric flow rate. Experiments were performed with refrigerant FC-72 which spanned the following conditions: initial pressure of Pin = 73 kPa, mass velocity of G = 94 – 333 kg/m2s, outlet quality of xe,out = 0 – superheat and heat flux of q″= 25–140 W/cm2. The result showed that, the maximum heat flux achieved 96 W/cm2, as the heating surface temperature was kept below 85 °C and critical heat flux occurred in the condition of low flow rate. Average two-phase heat transfer coefficients increased with the heat flux at low mass flux (G = 94 and 180 kg/m2s) and all heat fluxes, high mass flux (G = 333 kg/m2s) and all heat fluxes, and moderate mass fluxes (G = 224kg/m2s) under low and moderate heat fluxes (q″<110 W/cm2 for G = 224 kg/m2s), which was a feature of nucleate boiling mechanism. Pressure drop through microchannels heat sink was found to be below 4kPa.

Enhanced Flow Boiling Using Radial Open Microchannels With Manifold and Offset Strip Fins

2017 ◽  
Vol 140 (2) ◽  
Author(s):  
Alyssa Recinella ◽  
Satish G. Kandlikar
Keyword(s):  
Heat Flux ◽  
Pressure Drop ◽  
Flow Rate ◽  
Flow Boiling ◽  
Heat Fluxes ◽  
Valuable Insight ◽  
Reduced Pressure ◽  
Maximum Heat ◽  
Wall Superheat ◽  
Maximum Heat Flux

The increasing demand for designing effective cooling solutions in high power density electronic components has resulted in exploring advanced thermal management strategies. Over the past decade, phase-change cooling has received widespread recognition due to its ability to dissipate large heat fluxes while maintaining low temperature differences. In this paper, a radial flow boiling configuration through a central inlet was studied. This configuration is particularly suited for chip cooling application. Two heat transfer surfaces with (a) radial microchannels, and (b) offset strip fins were fabricated and their flow boiling performance with distilled water was obtained. Furthermore, the effect of the liquid flow rate on the boiling performance and enhancement mechanisms was also investigated in this study. At a flow rate of 240 mL/min, a maximum heat flux of 369 W/cm2 at a wall superheat of 49 °C and a pressure drop of 59 kPa was achieved with the radial microchannels, while the offset strip fins achieved a maximum heat flux of 618 W/cm2 at a wall superheat of 20 °C. Increasing the flow rate to 320 mL/min resulted in a heat flux of 897 W/cm2 demonstrating the potential of using a radial configuration for enhancing the boiling performance. The increase in flow cross-sectional area was shown to be responsible for the reduced pressure drop when compared to straight microchannel configurations. The high-speed imaging incorporated in each test provided valuable insight and understanding into the flow patterns and underlying mechanism in these geometries. With the ease of implementation, highly stable flow, and further optimization possibilities with different microchannel and taper configurations, the radial geometry is expected to provide significant performance enhancement well beyond a critical heat flux (CHF) of 1 kW/cm2.

Dryout Heat Fluxes for Inductively Heated Particulate Beds

1977 ◽  
Vol 99 (2) ◽  
pp. 250-256 ◽  
Author(s):  
V. Dhir ◽  
I. Catton
Keyword(s):  
Heat Flux ◽  
Flow Rate ◽  
Gravitational Force ◽  
Heat Fluxes ◽  
Distilled Water ◽  
Hydrodynamic Models ◽  
Gas Jets ◽  
Bed Height ◽  
Dry Out ◽  
Frequency Current

Experimental observations of the dryout heat fluxes for inductively heated particulate beds have been made. The data were obtained when steel and lead particles in the size distribution 295–787 microns were placed in a 4.7-cm dia pyrex glass jar and inductively heated by passing radio frequency current through a 13.3-cm dia multiturn work coil encircling the jar. Distilled water, methanol and acetone were used as coolants in the experiments, while the bed height was varied from 1.9 to 8.9 cm. Different mechanisms for the dryout in deep and shallow beds have been identified. Dryout in shallow beds is believed to occur when the vapor velocity in the gas jets exceeds a certain critical velocity at which choking of the vapor, leading to obstruction in the flow of the liquid towards the bed occurs. However, deep beds dry out when gravitational force can no longer maintain a downward coolant flow rate necessary to dissipate the heat generated in the bed. Finally, the heat flux data of the present investigation and that from two previous investigations made at Argonne Laboratory and at UCLA have been correlated with semitheoretical correlations based on the proposed hydrodynamic models.

Radiation and Convection Heat Flux Sensor for High Temperature Gas Environment

1998 ◽  
Author(s):  
Nelson Martins ◽  
Maria da Graça Carvalho ◽  
Naim Afgan ◽  
Alexander Ivanovich Leontiev
Keyword(s):  
Heat Flux ◽  
High Temperature ◽  
Mass Flow Rate ◽  
Mass Flow ◽  
Flow Rate ◽  
Critical Mass ◽  
Flux Measurement ◽  
Heat Fluxes ◽  
Sensing Element ◽  
Porous Element

The heat flux measurement is one of the essential parameter for the diagnostic of thermal systems. In the high temperature environment there are difficulties in differentiating between the convective and radiation component of heat flux on the heat transfer surface. A new method for heat flux measurement is being developed using a porous sensing element. The gas stream flowing through the porous element is used to measure the heat received by the sensor surface exposed to the hot gas environment and to control whether or not the sensing element receives the convection component of the total heat flux. It is possible to define a critical mass flow rate corresponding to the destruction of the boundary layer over the sensing element. With subcritical mass flow rate the porous sensing element will receive both the convective and radiative heat fluxes. A supercritical mass flow rate will eliminate the convective component of the total heat flux. Two consecutive measurements considering respectively a critical and a sub-critical mass flow rate can be used to determine separately the convection and radiation heat fluxes. A numerical model of sensor with appropriate boundary condition has been developed in order to perform analysis of possible options in the design of the sensor. The analysis includes: geometry of element, physical parameters of gas and solid and gas flow rate through the porous element. For the optimal selection of the relevant parameters an experimental set-up was designed, including the sensor element with corresponding cooling and monitoring system and high temperature radiation source. Applying the respective measuring procedure the calibration curve of the sensor was obtained. The linear dependency of the heat flux and respective temperature difference of the gas was verified. The accuracy analysis of the sensor reading has proved high linearity of the calibration curve and accuracy of ± 5%.

scholarly journals A Study of Critical Heat Flux During Flow Boiling in Microchannel Heat Sinks

2011 ◽  
Author(s):  
Tailian Chen ◽  
Suresh V. Garimella
Keyword(s):  
Heat Flux ◽  
Critical Heat Flux ◽  
Flow Rate ◽  
Heat Input ◽  
Heat Sink ◽  
Flow Boiling ◽  
Strong Dependence ◽  
Heat Fluxes ◽  
Abrupt Decrease ◽  
Parallel Microchannels

The cooling capacity of two-phase transport in microchannels is limited by the occurrence of critical heat flux (CHF). Due to the nature of the phenomenon, it is challenging to obtain reliable CHF data without causing damage to the device under test. In this work, the critical heat fluxes for flow boiling of FC-77 in a silicon thermal test die containing 60 parallel microchannels were measured at five total flow rates through the microchannels in the range of 20–80 ml/min. CHF is caused by dryout at the wall near the exit of the microchannels, which in turn is attributed to the flow reversal upstream of the microchannels. The bubbles pushed back into the inlet plenum agglomerate; the resulting flow blockage is a likely cause for the occurrence of CHF which is marked by an abrupt increase in wall temperature near the exit and an abrupt decrease in pressure drop across the microchannels. A database of 49 data points obtained from five experiments in four independent studies with water, R-113, and FC-77 as coolants was compiled and analyzed. It is found that the CHF has a strong dependence on the coolant, the flow rate, and the area upon which the flux definition is based. However, at a given flow rate, the critical heat input (total heat transfer rate to the coolant when CHF occurs) depends only on the coolant and has minimal dependence on the details of the microchannel heat sink (channel size, number of channels, substrate material, and base area). The critical heat input for flow boiling in multiple parallel microchannels follows a well-defined trend with the product of mass flow rate and latent heat of vaporization. A power-law correlation is proposed which offers a simple, yet accurate method for predicting the CHF. The thermodynamic exit quality at CHF is also analyzed and discussed to provide insights into the CHF phenomenon in a heat sink containing multiple parallel microchannels.

Near Critical Heat Flux From Small Substrates Under Controlled Spray Cooling

2009 ◽  
Author(s):  
Sergio Escobar-Vargas ◽  
Jorge E. Gonzalez ◽  
Orlando Ruiz ◽  
Cullen Bash ◽  
Ratnesh Sharma ◽  
...  
Keyword(s):  
Heat Flux ◽  
Mass Flow Rate ◽  
Critical Heat Flux ◽  
Mass Flow ◽  
Flow Rate ◽  
Heat Dissipation ◽  
High Heat ◽  
Heat Fluxes ◽  
Experimental Results ◽  
High Heat Flux

The increasing power density on electronic components has resulted in temperature problems related to the generation of hot spots and the need to remove high heat flux in small areas. This work is aimed at the cooling of small surfaces (1 mm × 1.2 mm) by using a monodisperse spray from thermal ink jet (TIJ) atomizers. Heat fluxes near the critical heat flux (CHF) are obtained for different conditions of cooling mass flow rate, droplet deposition, and number of active droplet jets. Experimental results at quasiequilibrium show the heat flux scales to the cooling mass flow rate. It is observed that two simultaneously activated jets result in slightly smaller heat flux compared to a single jet of droplets for the same mass flow rate. Droplet momentum and spreading or splashing, as determined by a combination of Weber number and Reynolds number effect via K = We1/2Re1/4, may impact the efficiency of the delivery of the cooling mass flow. Current experimental results at K = 24.5 and K = 52.2 for the copper surface temperatures ranging 110 – 120 °C indicate there is little influence of the splashing on the heat dissipation. System heat losses are measured experimentally and compared to a numerical and analytical solution to estimate the actual heat dissipated by the droplet change of phase.

scholarly journals Assessing IDDES-Based Wall-Modeled Large-Eddy Simulation (WMLES) for Separated Flows with Heat Transfer

Fluids ◽  
2021 ◽  
Vol 6 (7) ◽  
pp. 246
Author(s):  
Rozie Zangeneh
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Large Eddy Simulation ◽  
Skin Friction ◽  
Heat Fluxes ◽  
Separated Flows ◽  
Detached Eddy Simulation ◽  
Eddy Simulation ◽  
Large Eddy ◽  
Isothermal Wall

The Wall-modeled Large-eddy Simulation (WMLES) methods are commonly accompanied with an underprediction of the skin friction and a deviation of the velocity profile. The widely-used Improved Delayed Detached Eddy Simulation (IDDES) method is suggested to improve the prediction of the mean skin friction when it acts as WMLES, as claimed by the original authors. However, the model tested only on flow configurations with no heat transfer. This study takes a systematic approach to assess the performance of the IDDES model for separated flows with heat transfer. Separated flows on an isothermal wall and walls with mild and intense heat fluxes are considered. For the case of the wall with heat flux, the skin friction and Stanton number are underpredicted by the IDDES model however, the underprediction is less significant for the isothermal wall case. The simulations of the cases with intense wall heat transfer reveal an interesting dependence on the heat flux level supplied; as the heat flux increases, the IDDES model declines to predict the accurate skin friction.

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