Heater Size Research Articles

Effect of heated zone size on micro and nanoscale convective heat transfer

Recent studies on free convective heat transfer from micro and nanoscale structures show that the heat transfer coefficient is size dependent. The heat transfer coefficient is found to increase at smaller scales, which is ascribed to the higher surface area to volume ratio. Also, the mode is changed from advection to conduction due to the decaying influence of the gravitational field. Interestingly, it is tacitly assumed that the size effect is due to the specimen, since not a single study on the effect of the size of heat source on the convective heat transfer exists in the literature. In this study, we provide unambiguous experimental evidence of the predominance of heater size on the heat transfer coefficient. For micro-heaters, the heat transfer coefficient is measured to about 3200W/m2K. This value drops to about 110W/m2K for a millimeter scale heater and to 10W/m2K for a macroscopic heater; all for a nanoscale thin film specimen. This finding is particularly significant in microelectronic applications where localized heating (hot spots) in small areas is very common.

Numerical Investigation of Saturated Flow Boiling in Microchannels by the Lattice Boltzmann Method

Bubble formation in saturated flow boiling in 2D microchannels, generated from a microheater under constant wall heat flux or constant wall temperature conditions, is studied numerically based on a newly developed lattice Boltzmann model for liquid-vapor phase change. Simulations are carried out to study effects of inlet velocity, contact angle, and heater size on saturated flow boiling of water under constant wall heat flux conditions. Important information, such as effects of static contact angle on nucleation time and nucleation temperature, which was unable to be obtained by other numerical simulation methods, is obtained. Furthermore, effects of inlet velocity, contact angle, and superheat on nucleate boiling heat transfer in steady flow boiling of water under constant wall temperature conditions are also presented. It is found that the nucleate boiling heat transfer at the microheater is higher if the heater surface is more hydrophilic, because the superheated vapor at the hydrophilic wall has a thinner thermal boundary layer and a larger thermal conductivity.

On the thermo-economic optimisation of multistage flash evaporation desalination plants

A thermo–economic optimisation analysis is presented yielding simple algebraic formulae for estimating the optimum number of desalination stages, condensing unit and brine heater size with operating temperatures for distilled water production. The P1–P2 method is used in the present study, together with the thermo–economic analyses of desalination equipments.

Cantilever-free thermal actuation

The authors report the characterization and optimization of thermal actuators based upon thin elastomeric films on glass slides with integrated microheaters. By performing a systematic study of actuation performance with respect to heater size and elastomeric film thickness, the relationships between these parameters and actuation speed, efficiency, and crosstalk are elucidated. Combining these experimental studies with calculated temperature profiles provides an estimate of the maximum attainable actuation, which is predicted to be as large as 20% of the elastomeric film thickness. Based on these results, the authors provide a strategy for optimizing actuator geometry for a desired application in terms of selected actuation range and temperature tolerance. These results can be used to explore the feasibility of applying thermal actuation in a massively parallel format in low-cost microelectromechanical systems for applications such as high throughput, individually addressable cantilever-free scanning probe lithography.

An Experimental Research on a Phase Change Heat-Storage-Type Heat Pump Water Heater

An advanced experimental rig for heat pump water heaters was established. The performances of both a phase change heat-storage-type heat pump water heater (Type A) and a conventional heat pump water heater (Type B) were tested and compared according to GB/T 23137-2008 using the experimental rig. The results showed that the existence of phase transition temperature platform enabled the phase change heat-storage-type heat pump heater run longer and more steadily. When the same hot water heat capacity obtained, the volume of the accumulator in the Type A water heater accounted for about 56.9% of that of the water tank in the Type B water heater, which could save the overall size of water heater significantly and therefore promote the wide application of energy-saving heat pump water heaters.

The Heater Size and Viscosity Effects on Thermal Bubble Growth and Dynamics after Departure

A single bubble growth after boiling in square vertical tube is studied numerically to better understand bubble formation factors and as well as its moving behavior after departure. The effects of heating surface area and viscosity of fluid are investigate with nucleation time, bubble departure time, bubble dimension as well as boiling curve. The study indicate that the larger heating surface area, the smaller bubble departure diameter and higher nucleation frequency. The larger of viscosity of fluid, the smaller moving deviation from central axis after bubble departure and smaller deformation of the bubble.

Numerical study of the heater length effect on the heating of a solid circular obstruction centered in an open cavity

The steady vortex formation and thermal behavior in a two-dimensional square ventilated cavity is numerically studied. The governing equations of mass, momentum and energy are solved with a finite element method combined with an operator splitting scheme. We analyze the flow occurring inside the enclosure with a centered circular obstruction and a heater plate located at the center of the lower wall of the cavity. The size of the heater is varied for five different lengths. The simulations are obtained for Richardson and Prandtl numbers of 0.01 to 10.0 with a Reynolds number of 400. Results are reported in the form of streamlines, isotherms, average Nusselt number, average bulk fluid and obstruction temperatures. The effects of the heater length, Richardson number and Prandtl number on the hydrodynamics and thermal behavior have been investigated. Four vortex formation mechanisms are identified: (a) the inertial effect of the inlet jet which moves from the lower left sidewall to the upper right sidewall, (b) the detachment of the boundary layer from the wall located above of the entrance of the cavity and from the obstruction, (c) the rolling up of the fluid when this meets with the corners of the cavity and (d) the thermal boundary layer formed on the heater which originates a thermal plume with instabilities traveling upward. It is observed that for low Prandtl numbers with large heater sizes and high Richardson numbers the temperature of the obstruction is increased.

Laminar natural convection in inclined rectangular cavities with a localized heat source

The paper investigates numerically laminar natural convection in inclined rectangular cavities with a localized heat source. A mathematical model was constructed where the conservation equations governing the mass, momentum, and thermal energy together with their boundary conditions were solved. The calculation grid used in the solution is investigated to determine the best grid spacing, the required number of iterations, and other parameters which affect the accuracy of the generated solutions. The numerical method and computer program were tested for the case of pure conduction to assure validity and accuracy of the numerical method.The numerical investigation used air as the fluid and covered Rayleigh numbers based on scale length, s/A ranging from 102 to 106, aspect ratio from 0.5 to 5, position ratio from 0.25 to 0.75, heater size ratio from 0.25 to 1, and the tilt angle measured from horizontal was varied from 0 to 180°. The results are presented graphically in the form of streamline and isotherm contour plots. The heat transfer characteristics, velocity profiles, local and average Nusselt numbers were also presented. A correlation was developed which represents the present numerical heat transfer results with an average deviation of less than 11.5%.

Effect of heater size and Reynolds number on the partitioning of surface heat flux in subcooled jet impingement boiling

The overall heat transfer rate due to boiling of an impinging subcooled liquid jet is attributed to several simultaneous mechanisms including liquid and vapor phase convection, quenching (transient convection) and evaporation. In the present research, the Rensselaer-Polytechnic Institute (RPI) wall-boiling model is employed to carry out computational simulations on subcooled turbulent jet impingement boiling of water in a confined and submerged configuration. The effects of the controlling parameters, viz. heater-nozzle size ratios (wH/wN), degree of surface superheats (ΔTsat), jet-Reynolds numbers (Re) are studied on the partitioning of the total heat flux into liquid phase convection, quenching (transient convection) and evaporation. The effects of these controlling parameters were studied in the ranges 1⩽wH/wN⩽11, −5°C⩽ΔTsat⩽25°C (heat fluxes upto 200W/cm2) and Re=2500 and 3750, for isothermal and isoflux heaters. The present computational approach was validated by comparison of the local temperature distribution on the heater, and boiling curves against experimental data in literature for axisymmetric and slot jets. The liquid phase convective and quenching components of the total heat flux were found to be larger for relatively lower values of wH/wN, implying smaller heaters. On the contrary, the evaporative counterpart was found to be smaller for relatively smaller heaters upto about wH/wN=3, beyond which the change was negligible with increase in wH/wN. Beyond a threshold surface superheat, the heat flux due to quenching was found to be the largest contributor to the total heat flux; and this threshold value of surface superheat decreased almost exponentially with increase in heater size. It was also observed that irrespective of the heater size, the liquid phase convective component of the total heat flux for Re=3750 was consistently larger as compared to Re=2500, while the quenching and evaporative components of the total heat flux were nearly equal for both Reynolds numbers. An artificial neural network (ANN) was trained and tested with data sets generated from the present research, and ready-to-use weights are presented.

Convective transport in rectangular cavities partially heated at the bottom and cooled at one side

Laminar natural convection heat transfer inside air-filled, rectangular enclosures partially heated from below and cooled at one side is studied numerically. A computational code based on the SIMPLE-C algorithm is used for the solution of the system of the mass, momentum, and energy transfer governing equations. Simulations are performed for a complete range of heater size, for Rayleigh numbers based on the height of the cavity ranging from 10~3to 10~6, and for height-to-width aspect ratios of the cavity spanning from 0.25 to 4. It is found that the heat transfer rate increases with increasing the heater size and the Rayleigh number, while it decreases with increasing the aspect ratio of the cavity. Dimensionless heat transfer correlations are also proposed.

Still Too Hot

Although water heater manufacturers adopted a voluntary standard in the 1980s to preset thermostats on new water heaters to 120°F, tap water scald burns cause an estimated 1500 hospital admissions and 100 deaths per year in the United States. This study reports on water temperatures in 976 urban homes and identifies water heater and household characteristics associated with having safe temperatures. The temperature of the hot water, type and size of water heater, date of manufacture, and the setting of the temperature gauge were recorded. Demographic data, including number of people living in the home and home ownership, were also recorded. Hot water temperature was unsafe in 41% of homes. Homeowners were more likely to have safer hot water temperature (<120°F) than renters (63 vs 54%; P < .01). For 11% of gas water heaters, the water temperature was >130°F, although the gauge was set at less than 75% of its maximum setting. In a multivariate logistic regression, electric water heaters were more likely to have safe hot water temperatures than gas water heaters (odds ratio R=4.99; P < .01). Water heaters with more gallons per person in the household were more likely to be at or below the recommended 120°F. Our results suggest that hot water temperatures remain dangerously high for a substantial proportion of urban homes despite the adoption of voluntary standards to preset temperature settings by manufacturers. This research highlights the need for improved prevention strategies, such as installing thermostatic mixing valves, to ensure a safer temperature.

발열장치를 이용한 기능성 스마트 파운데이션의 구성 시안

This research was intended to design an experimental girdle with thermal insulation function for adult women in their 20s. The design of the experimental girdle was based on the pattern of commercially available girdle. The final pattern of the experimental girdle was established according to the drawing equations determined based on the result of appearance evaluation. The equations were (waist circumference<TEX>${\times}0.88$</TEX>)/2 for waist circumference, (hip circumference<TEX>${\times}0.77$</TEX>)/2 for hip circumference, and (thigh circumference<TEX>${\times}0.85$</TEX>) for thigh circumference. In order to develop a heating device, the most effective fabric heater was adopted based on the experiments about the number of caron fibers, heater size and attachment site. Three heaters-one with a size of <TEX>$14.5{\times}9.5$</TEX> cm, and the other two with the size of <TEX>$8.0{\times}15.0$</TEX> cm-were attached to the areas corresponding to the lower abdomen and the hip, 5 cm below the waist. A heater was developed by connecting these heaters to a controller, 2 batteries (7.4 V 2000 mAh lithium polymer batteries) and a switch (for mode conversion between high/medium/low temperatures). The heater was integrated into the inside of the girdle, so that attachment and detachment were possible without the change of appearance. The tentative configuration plan was proposed for the development of a functional smart girdle with an excellent thermal insulation effect.

Pool Boiling Heat Transfer on the International Space Station: Experimental Results and Model Verification

The relatively poor understanding of gravity effects on pool boiling heat transfer can be attributed to the lack of long duration high-quality microgravity data, g-jitter associated with ground-based low gravity facilities, little data at intermediate gravity levels, and a poor understanding of the effect of important parameters even at earth gravity conditions. The results of over 200 pool boiling experiments with n-perfluorohexane as the test fluid performed aboard the International Space Station (ISS) are presented in this paper. A flat, transparent, constant temperature microheater array was used to perform experiments over a wide range of temperatures (55 °C &lt; Tw &lt; 107.5 °C), pressures (0.58 atm &lt; P &lt; 1.86 atm), subcoolings (1 °C ≤ ΔTsub ≤ 26 °C), and heater sizes (4.2 mm ≤ Lh ≤ 7.0 mm). The boiling process was visualized from the side and bottom. Based on this high quality microgravity data (a/g&lt;10−6), the recently reported gravity scaling parameter for heat flux, which was primarily based on parabolic flight experiments, was modified to account for these new results. The updated model accurately predicts the experimental microgravity data to within ±20%. The robustness of this framework in predicting low gravity heat transfer is further demonstrated by predicting many of the trends in the pool boiling literature that cannot be explained by any single model.

Numerical simulation of natural convection in a vertical annulus with a localized heat source

The present numerical investigation deals with the size and location effects of a single isoflux discrete heater on the buoyancy induced convection in a cylindrical annulus. A discrete heater is placed at the inner wall, while the top and bottom walls as well as the unheated portions of the inner wall are kept adiabatic, and the outer wall is maintained at a lower temperature. The influence of location and size of the discrete heater on the convective flow and the corresponding heat transfer are obtained for a wide range of physical parameters. The predicted numerical results reveal that the placement of heater near the middle portion of inner wall yields a maximum heat transfer and minimum hot spots rather than placing the heater near the top and bottom portions of the inner wall. We found that the location of heater affects the rates of flow circulation and heat transfer in a complex fashion. The rate of heat transfer is an increasing function of radii ratio of the annulus. Further, we found that the rate of heat transfer and maximum temperature in the annular cavity are significantly modified by the heater length and location.

A theoretical CHF model for downward facing surfaces and gaps under saturated boiling

A theoretical model is developed to predict the critical heat flux (CHF) based on the description of the hydrodynamic behavior of the vapor-liquid interface of a bubble at the heater surface leading to the initiation of CHF condition under saturated boiling. The CHF model considering heater size and gap size effects is developed to account for surface orientation effect, gap size effect, pressure effect, and contact angle effect for downward facing surfaces and gaps with orientations of 0° (horizontal downward facing position) to 90° (vertical position). The CHF in pool boiling and gap boiling was well predicted by the model for different effects. And the CHF model was extended to predict the CHF of the reactor-scaling hemispherical surface. The results indicated that the CHF model could also well predict the experimental data on large scale hemispherical surface. The present work is instructive for the safety analysis of the lower head of PWR in case of core meltdown during severe accident.

Numerical Parameter Study of Low-Electric-Power Segmented Arc Heaters

The effect of arc heater configuration and input operational conditions on arc heater flows is investigated to design of a low electric power segmented arc heater through numerical simulations. The existing computer code ARCFLO4 to solve the segmented arc heater flows is used for this numerical study. First, the performance change caused by the arc heater configuration is observed as a constrictor length, a constrictor diameter, an electrode chamber diameter, a cathode chamber length, and a nozzle throat diameter arc changed. Afterward, the performance change caused by an input current and a mass flow rate is investigated for a specific arc heater configuration. From the study, it is confirmed that the change of the mass averaged enthalpy is related more deeply to the constrictor diameter than the constrictor length and the stability of arc heat operation becomes higher as the arc column and the plasma column are lager. Also, it is found that the flow separation in the cathode chamber affects the settling chamber pressure. On the one hand, it is confirmed that proper input operational conditions have to be determined to stabilize arc heater operation as well as configuration. Finally, a 750kW arc heater size is determined and its operational range is found successfully.

In situ thermal diffusivity determination of anisotropic composite structures: Transverse diffusivity measurement

An in situ transient method has been developed for determining transverse thermal diffusivity (αz) of anisotropic composite shells under close to ambient conditions. The problems posed by anisotropy, large thicknesses, linear losses due to convection and radiation and unavoidable lateral conduction loss due to partial heating of large structures have been addressed. The basic approach proposed here is to select appropriate heater size, so that the characteristic time for in-plane conduction is always large compared to the record length (i) preferably of the entire experiment or (ii) at least of the rising part of the time–temperature curve recorded on the rear surface, following an appropriate square pulse excitation. This has ensured adherence to one dimensional heat propagation condition in spite of (i) anisotropy, (ii) partial heating of surface and (iii) long time of transit through thicker shells. Linear losses have been corrected by standard methods available in the literature. Finally, this in situ technique has been validated with simulation and experiments on cylindrical cfrp structures of 5–15mm thickness.

Visualization study of critical heat flux mechanism on a small and horizontal copper heater

We examined the influence of the macrolayer under a large vapor mushroom on critical heat flux (CHF) during pool boiling with a small plate heater. Evidence of a macrolayer was provided, and its measured thickness was compared and well agreed with values reported in the literature. The classical CHF models of pool boiling do not consider the effect of the heater size. For a small and horizontal heater, a hydrodynamic liquid inflow increases the CHF beyond the predictions of most models, which are based on an infinite and horizontal heater. Using high speed visualization of CHF, we proposed a new CHF triggering mechanism for a small heater. Because the hydrodynamic liquid inflow supplies liquid to the edge of the heater, nucleate boiling is maintained in this region, even when a large dry patch beneath the large mushroom is generated at the center of the heater. The CHF occurs when the macrolayer at the edge of the heater dries out (i.e., becomes coated with a vapor film) and meets the large dry patch at the center of the heater. Finally, we proposed the shape of macrolayer beneath the large mushroom in order to explain the CHF triggering mechanism on the small heater.

Program and Read Scaling Trade-Offs in Phase Change Memories

This paper investigates the scaling perspectives of Phase Change Memory technology by analyzing the effects of geometrical dimensions reduction and the read voltage scaling on program and read operations. To this end, we derive analytical expressions which allow us to estimate the dependence of the RESET and read current on key geometrical parameters of the memory cell, such as heater size and chalcogenide layer thickness.

On the Scaling of Pool Boiling Heat Flux With Gravity and Heater Size

A framework for scaling pool boiling heat flux is developed using data from various heater sizes over a range of gravity levels. Boiling is buoyancy dominated for large heaters and/or high gravity conditions and the heat flux is heater size independent. The power law coefficient for gravity is a function of wall temperature. As the heater size or gravity level is reduced, a sharp transition in the heat flux is observed at a threshold value of Lh/Lc = 2.1. Below this threshold value, boiling is surface tension dominated and the dependence on gravity is smaller. The gravity scaling parameter for the heat flux in the buoyancy dominated boiling regime developed in the previous work is updated to account for subcooling effect. Based on this scaling parameter and the transition criteria, a methodology for predicting heat flux in the surface tension dominated boiling regime, typically observed under low-gravity conditions, is developed. Given the heat flux at a reference gravity level and heater size, the current framework allows the prediction of heat flux at any other gravity level and/or heater size under similar experimental conditions. The prediction is validated using data at over a range of subcoolings (11 °C ≤ ΔTsub ≤ 32.6 °C), heater sizes (2.1 mm ≤ Lh ≤ 7 mm), and dissolved gas concentrations (3 ppm ≤ cg ≤ 3500 ppm). The prediction errors are significantly smaller than those from correlations currently available in the literature.