Effects Of Heater Size Research Articles

Effect of combined natural convection and non-gray gas radiation on entropy generation in a circular enclosure with partial heating

This article reports a numerical analysis of combined natural convection and non-gray gas radiation within a cylindrical enclosure, isothermally heated and cooled on various arc lengths of the sidewall. Three active zone locations are studied in this article. The first heating section extends quarter the perimeter of the cavity, the second spreads across one-half of the cylinder, and the third one is made of three-quarters of the sidewall. The participating media are considered as emissive, absorbent, and non-scattering. The radiative transfer equation is resolved using the Ray-Tracing method associated to the Statistical Narrow Band–correlated K model. The effect of heater size and its location on heat transfer, fluid flow, and entropy generation are presented and discussed in this work. The results show that the optimum heater size is obtained for three-quarters heated enclosure. It was also found that choosing a heat source centered at the top of the enclosure provides the best heat transfer performance.

Experimental study of sound emission in subcooled pool boiling on a small heating surface

In this study, sound emission in subcooled pool boiling with a small plate heater was investigated experimentally. Acoustic signals were recorded using a hydrophone and analyzed using sound pressure level (SPL), fast Fourier transform, and discrete wavelet transform (DWT). The results show that the SPL combined with an analysis of the sound signal by DWT can be used to identify boiling modes and heat transfer intensification. In contrast to those for a large plate heater or long wire, the plot of the SPL against wall superheat in nucleate boiling region exhibits an ‘N” shape. Visualization results indicate that this difference mainly originates from the effects of heater size on bubble growth and departure. Compared with the nucleate boiling, in the region of microbubble emission boiling (MEB), the SPL exhibits a step increase of 10–20 dB, and continues to increase with the further increase of the heat flux. A comparison of the bubble frequency and features of the acoustic signals shows that the loud boiling sound in the MEB could be mainly attributed to the vapor film collapse and vapor volume oscillations, which also significantly affect the heat transfer process.

Effect of angle of applied magnetic field on natural convection in an open ended cavity with partially active walls

In this work, 2D, steady, natural convection in a partially active/heated open ended square cavity subjugated to the magnetic field for incompressible, Newtonian fluid is studied and presented. A simplified double distribution function (DDF)-thermal lattice Boltzmann method (TLBM) based on single relaxation time (SRT) is utilized for solving field controlling equations. One of the vertical walls of open cavity is exposed to the heat source (i.e., heater) partially; while another vertical wall is open to ambient. In particular, the influence of various geometric as well as parametric conditions, such as heater size (LH=0.25, 0.5, 0.75), angle of magnetic field (θM=0°, 45° and 90°), Hartmann number (0≤Ha≤100) and Rayleigh number (103≤Ra≤105) on local and global convection features have been investigated. The dependence of average Nusselt number with magneto-convective parameter (λ=Ha2/Ra) has also been illustrated. It is observed that cavity with the applied magnetic field at θM=45° offers highest heat transfer restriction than other considered cases. Effect of heater size remains effective only at higher Ra (≥105).

Mesoscale simulations of saturated pool boiling heat transfer under microgravity conditions

Microgravity effects on pool boiling heat transfer from a horizontal hydrophilic surface under constant wall temperature are simulated numerically based on an improved liquid-vapor phase-change lattice Boltzmann method with the imposition of a conjugate thermal boundary condition at solid/liquid interface. Effects of microgravity on bubble departure diameter and bubble departure frequency are investigated. Both the bubble departure diameter and the duration of one bubble cycle are found to be increasing with decreasing gravity levels. Boiling curves under normal gravity and microgravity are numerically obtained and compared. It is shown that gravity has significant effects on pool boiling curves from nucleate boiling to critical heat flux, and from transition boiling to film boiling heat transfer. Moreover, the critical heat flux is lower and it occurs at a lower wall superheat under microgravity conditions than under normal gravity. Effects of heater size in pool boiling under both of microgravity and normal gravity conditions are investigated as well.

Mesoscale simulation of heater size and subcooling effects on pool boiling under controlled wall heat flux conditions

Effects of heater size and subcooling on pool boiling from horizontal heated hydrophilic and hydrophobic surfaces under controlled wall heat flux conditions are numerically simulated with a new scheme of phase-change lattice Boltzmann method. Simulation results confirm that the shape of saturated pool boiling curves depends greatly on the ratio of heater length (LH) to the capillary length (l0). For large heaters with LH/l0⩾12, boiling curves collapse into one and the heater can be considered to be of infinite size. For heater sizes in the range of 4<LH/l0<12, the heater size has great influence on CHF, transition boiling and film boiling regimes. It was found that the CHF increases with the decreasing heater size and existing correlation equations on heater size effects on critical heat flux are valid. For small heaters with LH/l0⩽4, the transition boiling regime disappears, and the nucleate boiling mode switches to the film boiling mode immediately as the wall heat flux is increased. It is also shown that subcooling has relatively small effects in natural convection, in the nucleate boiling regime and film boiling regime, but it enhances CHF greatly and shorten the transition boiling regime. The relationship between simulated critical heat fluxes of saturated and subcooled nucleate boiling in pool boiling agrees well with existing correlation equations.

Experimental study of the heater size effect on subcooled pool boiling heat transfer of FC-72 in microgravity

Experiments of highly subcooled nucleate pool boiling of FC-72 with dissolved air were conducted on a large scale silicon chip (2×2×0.05cm3, denoted as chip S 2×2) in short-term microgravity condition and normal gravity condition by utilizing the drop tower in Beijing respectively. The results were compared with published results of a smaller scale chip (1×1×0.05cm3, denoted as chip S 1×1) both in normal gravity condition and microgravity condition, to study the heater size effect on boiling heat transfer. It is indicated that in microgravity, the input heat flux range in which bubble departure took place for chip S 2×2 is wider than that for chip S 1×1, and an interesting phenomenon observed is that coalesced bubbles departed at a continuously smaller radii at a given heat flux. Initially, the average bubble departure radii for chip S 2×2 increased linearly with heat flux while later remained constant. At a same heat flux, chip S 2×2 showed a bubble departure radius larger than that of chip S 1×1. By comparison, it is found that nucleate boiling performance deteriorates with increase in heater size in both earth gravity condition and microgravity condition. However, in microgravity the qCHF of chip S 2×2 is 20% greater than that of chip S 1×1, contrary to the CHF characteristic in earth ground condition. Moreover, boiling patterns in microgravity are different in high heat flux region: a smooth hemispherical bubble was generated on chip S 1×1, while on chip S 2×2 an oblate vapor blanket was formed, which indicates that different dominated boiling heat transfer mechanisms exist in both cases. It is found that in microgravity, the boiling was dominated by buoyancy for chip S 2×2, but it was in surface tension dominated boiling regime for chip S 1×1, which proved the above speculation. Moreover, the range of transition heater size criteria is 1.45<LhLc<2.89 in the present work. By using the updated model developed by Raj and Kim, it is discovered that predicted values are generally lower than experimental values in both experiments. Moreover, differences were more evident for chip S 2×2.

Effect of heater size on ultrasonic enhancement of boiling in water and surfactant solutions

Experiments were performed to study enhancement of heat transfer from the wire of d = 50 µm and the tube of d = 1.5 mm in subcooled pool boiling by ultrasonic waves. The working fluids are clean water and Alkyl (8-16) Glucoside surfactant solutions of different concentrations and bulk temperature 30 °C. The wire resistance was translated to the temperature, using the calibration data, the temperature of the tube was measured by thermocouple. The differences between effect of ultrasonic field on boiling in water for heaters of d = 50 µm and d = 1.5 mm may be summarized as follows: for boiling on the wire of d = 50 µm in subcooled water, Tb = 30 °C, enhancement of heat transfer coefficient due to applied ultrasonic field is about 70% and 20% at heat flux q = 620 kW/m2 and q = 1350 kW/m2, respectively. For boiling in surfactant solutions at the same boiling conditions enhancement of heat transfer coefficient is in the range of 5–10% at heat flux q = 620 kW/m2 and 10–16% at heat flux q = 1350 kW/m2 depending on solution concentration. For boiling on the tube of d= 1.5 mm in subcooled water, Tb= 30 ℃, enhancement of heat transfer coefficient due to applied ultrasonic field is about 50% and 45% at heat flux q = 500 kW/m2 and q = 2500 kW/m2, respectively. The same values are obtained for boiling in surfactant solution of concentration C = 250 ppm. For the wire of d = 50 µm the heat transfer enhancement due to acoustic vibrations in surfactant solutions is not as strong as in water. This fact may be considered as evidence of significant role of relationship between jet flow and ultrasonic field.

Experimental study on the stagnation line heat transfer characteristics with high-velocity free slot jet impingement boiling

In this paper, the stagnation line heat transfer characteristics and critical heat flux (CHF) of small slot jet impingement boiling were experimentally investigated. Water with the Reynolds number range 34,013–136,054, the size of slot jet nozzle of 12mm×1mm, and the non-dimensional nozzle to target spacing of 0.2 were used. Besides, heat transfer surface sizes were 5mm×1mm and 1.5mm×1mm. It was found that the stagnation line heat transfer characteristics of the jet impingement nucleate boiling in the fully developed region are irrelevant to jet velocity, subcooling and heater size for the slot jet impingement in the present experimental range. The heat transfer correlation of nickel foil heater for the jet impingement nucleate boiling in the fully developed region was obtained. Meanwhile, the CHF of the jet impingement boiling on the stagnation line was also studied. Using the experimental data to modify the semi-empirical and semi-theoretical correlation proposed previously for predicting the CHF, its modified form with heater size effect was also obtained. Through increasing jet velocity and subcooling while decreasing heater size, extremely critical heat fluxes (>108W/m2) were achieved in the experiment, and its maximum value was 1.14×108W/m2. In addition to decreasing heater size to a moderate value, it is also necessary to change heater’s surface features including CA to achieve a higher heat flux.

A modified hydrodynamic model for pool boiling CHF considering the effects of heater size and nucleation site density

Boiling is commonly used in daily life and is an efficient heat removal method. During pool boiling, the critical heat flux (CHF) establishes the upper limit of efficient heat removal. Although, numerous studies have explored CHF, its exact mechanisms remain ambiguous. The hydrodynamic instability model is the most prominent CHF model, but it fails to explain the evident dependence of CHF on surface properties. The authors proposed modifying the hydrodynamic model to account for how nucleation site density and finite heater size affect the CHF. The experimental CHF values obtained from evaluating various types of surfaces qualitatively agreed with the predictions of the modified hydrodynamic model. This suggests that the CHFs on these surfaces were due to the hydrodynamic limit.

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.

Optimized Design of Heaters for Flying Height Adjustment to Preserve Performance and Reliability

We developed a thermal flying-height control (TFC) slider to control the flying height of magnetic recording heads. The slider basically consists of a small heater fabricated near the read/write element. This study discusses the effect of heater size and heater location on the change in the flying height at the read/write element. We also discuss the resulting temperature rise due to the additional heat applied by the heater. Specifically, we have found that small heaters generally resulted in lower heater power per unit change in the flying height and lower head temperature rise per unit change in the flying height. In terms of heater location, we have found that a heater closer to the air-bearing surface (ABS) also tends to result in a larger change in the flying height because of the larger protrusion shape. However, the head temperature rose significantly. Therefore, shorter ABS/heater distance was a trade off lower power against higher rise in head temperature. We concluded that smaller heaters and the shorter ABS/heater distance are better as long as head reliability is ensured

Parametric Study of Pool Boiling on Horizontal Highly Conductive Microporous Coated Surfaces

To better understand the mechanisms that govern the behavior of pool boiling on horizontal highly conductive microporous coated surfaces, a series of experimental investigations were designed to systematically examine the effects of the geometric dimensions (i.e., coating thickness, volumetric porosity, and pore size, as well as the surface conditions of the porous coatings) on the pool-boiling performance and characteristics. The study was conducted using saturated distilled water at atmospheric pressure (101kPa) and porous surfaces fabricated from sintered isotropic copper wire screens. For nucleate boiling on the microporous coated surfaces, two vapor ventilation modes were observed to exist: (i) upward and (ii) mainly from sideways leakage to the unsealed sides and partially from the center of porous surfaces. The ratio of the heater size to the coating thickness, the friction factor of the two-phase flow to single-phase flow inside the porous coatings, as well as the input heat flux all govern the vapor ventilation mode that occurs. In this investigation, the ratio of heater size to coating thickness varies from 3.5 to 38 in order to identify the effect of heater size on the boiling characteristics. The experimental results indicate that the boiling performance and characteristics are also strongly dependent on the volumetric porosity and mesh size, as well as the surface conditions when the heater size is given. Descriptions and discussion of the typical boiling characteristics; the progressive boiling process, from pool nucleate boiling to film boiling; and the boiling performance curves on conductive microporous coated surfaces are all systematically presented.

Study of buoyancy-induced flows subjected to partially heated sources on the left and bottom walls in a square enclosure

In this paper, numerical simulations of laminar, steady, two-dimensional natural convection flows in a square enclosure with discrete heat sources on the left and bottom walls are presented using a finite-volume method. Two different orientated wall boundary conditions are designed to investigate the natural convection features. The computational results are expressed in the form of streamlines and isothermal lines for Rayleigh numbers ranging from 10 2 to 10 7 in the cavity. In the course of study, a combination of third-order and exponential interpolating profile based on the convective boundedness criterion is proposed and tested against the partially heated cavity flow up to the highest Rayleigh number 10 7. The effects of thermal strength and heating length on the hydrodynamic and thermal fields inside the enclosure are also presented. Numerical results indicate that the average Nusselt number increases as Rayleigh number increases for both cases. Moreover, it is seen that the effect of the heat transfer rate due to the heating strength on the left wall is different from the one on the bottom. For the heater size effect, it is observed that by increasing the length of heat source segment, the heat transfer rate is gradually increased for both cases.

An experimental study of heater size effect on micro bubble generation

An experimental study of the heater size effect on micro boiling is reported in detail. Using a 1.66-ms-wide heating pulse, boiling in subcooled water was investigated on a series of micron/submicron thin film Pt heaters with various feature sizes ranging from 0.5 μm to 70 μm. It was found that there existed a critical heater size (10 μm): single spherical bubble generation with heater’s feature size less than 10 μm; oblate vapor blanket on the heater surface with the size larger than 10 μm. The bubble dynamics was studied by the visualization of the bubble nucleation process with a high-speed CCD. The onset bubble nucleation temperature was measured by using each Pt heater as a resistive temperature sensor. The formation of the oblate vapor blanket was attributed to the condensation effect of the vapor outside the superheated zone. The analysis was further validated by generating spherical bubble on heater with size larger than 10 μm with a longer heating pulse.

Allowable Repetition Frequency of Pulse Heating in Microactuators Using Rapid Boiling (Effect of Electrical Insulation Layer beneath Heater)

High-frequency pulse heating of heaters in microactuators utilizing the rapid expansion of boiling bubbles causes a gradual increase in the temperature of the heater and the substrate material in the immediate vicinity of the heater, resulting in a loss of reproducibility of the boiling phenomena. In the present report, the maximum frequency at which the steady increase in heater temperature can be maintained within the allowable limit for the succession of pulse heating is obtained by numerical simulations based on the heat conduction from the heater to the adjacent materials in the system having an electrical insulation layer between the heater element and the silicon substrate. Calculations are carried out using a model in an axisymmetrical system for two different heating conditions, that is, transient heating by a single pulse and steady heating by the time-averaged power during repetition of pulses, instead of the direct calculation of the entire repetition process. The effects of heater size and pulse width on the allowable repetition frequency are examined over wide ranges of these parameters. The influence of the insulation layer on allowable repetition frequency is investigated by comparing the calculation results with those of the case of without the insulation layer. Approximate correlations for the allowable repetition frequency are also derived based on an analytical examination.

Effects of heater size and working fluids on nucleate boiling heat transfer

The present study is an experimental investigation of nucleate boiling heat transfer mechanism in pool boiling from wire heaters immersed in saturated FC-72 coolant and water. The vapor volume flow rate departing from a wire during nucleate boiling was determined by measuring the volume of bubbles from the wire utilizing the consecutive-photo method. The effects of the wire size on heat transfer mechanism during a nucleate boiling were investigated, varying 25 μm, 75 μm, and 390 μm, by measuring vapor volume flow rate and the frequency of bubbles departing from a wire immersed in saturated FC-72. One wire diameter of 390 μm was selected and tested in saturated water to investigate the fluid effect on the nucleate boiling heat transfer mechanism. Results of the study showed that an increase in nucleate boiling heat transfer coefficients with reductions in wire diameter was related to the decreased latent heat contribution. The latent heat contribution of boiling heat transfer for the water test was found to be higher than that of FC-72. The frequency of departing bubbles was correlated as a function of bubble diameters.

Research on Micro Heater Efficiency for Thermal Ink Jet Head

Micro heaters for thermal ink jet print heads were investigated to estimate the effects of heater size and operating condition on its thermal efficiency. Tantalum aluminum (TaAl) heaters, 0.15 &#x3BC;m in thickness, were prepared on silicon oxide layer. Silicon nitride film of 0.4 &#x3BC;m thickness covered the heater. A visualization technique was applied to observe bubbles in open pool condition. Volume and lifetime of bubble were measured from CCD images of plane and side views of the bubble generated at the heat flux range of 0.5 &#x223C; 5.0 GW/m2. Thermal conduction simulation for the heater provided bubble nucleation pressure. Bubble work was calculated with Asai's pressure model (Ref. [2]) and the visualization results, bubble lifetime and volume. Then, thermal efficiency of the heater defined as the ratio of bubble work to the electric input energy could be estimated.

A study of the effects of heater size, subcooling, and gravity level on pool boiling heat transfer

Pool boiling heat transfer measurements from different heater sizes were obtained in low-g (0.01 ± 0.025 g) and high-g (1.7 ± 0.5 g) aboard the KC-135 aircraft. Boiling on three heaters of different size (0.65, 2.62, 7.29 mm 2) was studied. Control circuitry was used to maintain an isothermal boundary condition on the heater surface while the power dissipated by the heater was measured. Steady-state boiling data in low-g and high-g were obtained for various bulk fluid subcoolings and wall superheats. For the two larger heaters in low-g, critical heat flux (CHF) increased with increasing bulk subcooling and is significantly smaller than the CHF obtained in high-g. Satellite bubble coalescence with the primary bubble was the mechanism by which CHF occurred in low-g for larger heaters. At high subcoolings and wall superheats, above that of which CHF occurred, the onset of strong thermocapillary convection caused an increase in heat transfer above CHF. For the small heater in low-g, bulk subcooling had a negligible impact on boiling performance and a characteristically low heat flux was observed for the CHF. In high-g, boiling on heaters roughly the size of the bubble departure diameter was dominated by surface tension and characterized by a very low bubble departure frequency. For larger heaters in high-g, bubble departure frequency decreased with increasing subcooling but time and surface averaged heat transfer remained unaffected.

An Experimental Study of Miniature-Scale Pool Boiling

AbstractBy generating single bubbles on a micro-heater at different wall superheats, an experimental study of miniature-scale pool boiling heat transfer has been performed to provide a fundamental understanding of the heater size effect. In this study, the constant-temperature microheater is set at different temperatures by an electronic feedback control system. The heat transfer history during the lifetime of a single bubble which includes nucleation, growth, detachment and departure has been measured. The boiling curve obtained from the microheater is composed of two regimes which are separated by a peak heat flux. It is suggested that in the lower superheat regime, the boiling is dominated by liquid rewetting and micro-layer evaporation, while in the higher superheat regime, conduction through the vapor film and micro-convection plays the key heat transfer role as the heater is covered by vapor all the time. In general, boiling on microheaters is characterized by larger bubble departure sizes, smaller bubble growth rates due to the dryout of microlayer as the bubble grows, and higher bubble incipience superheat. As the heater size decreases, the boiling curve shifts towards higher heat fluxes with corresponding higher superheats.

Allowable Repetition Frequency of Pulse Heating in Microactuators Using Rapid Boiling (Effects of Heater Size and Substrate Material)

High frequency pulse heating of heaters in microactuators utilizing the rapid expansion of boiling bubbles causes a gradual increase in the temperature of the heater and the substrate material in the immediate vicinity of the heater, resulting in a loss of reproducibility of the boiling phenomena. In the present report, the maximum frequency at which the time-averaged increase in the heater temperature can be maintained within allowable limits (allowable frequency) is obtained from numerical simulations of heat conduction from the heater to adjacent materials using a model in an axisymmetrical system. Instead of direct calculation of the entire repetition process, calculations for two different heating conditions, single pulse heating and steady heating at a time-averaged power during repetition of pulses, are performed. The effects of heater size, pulse width and substrate properties on the allowable frequency are examined and approximate correlations for the allowable frequency are derived in dimensionless form based on an analytical examination. The results show that the allowable frequency increases significantly with a decrease in heater size for prescribed conditions of pulse width and temperature increases during both pulsed heating and steady heating at a time-averaged power. The simulated results are verified by comparison with those from experiments both with and without boiling.