Threshold Heat Flux Research Articles

Rapid bubble growth within 200 μs in flat microchannels under “hot spot” heating conditions at ultra-high heat fluxes

The two-phase flow boiling in microchannels is an attractive cooling strategy for high-power density electronic/optoelectronic devices, while the exist of sub-millimeter “hot spots” characterized by ultra-high heat fluxes would degrade the reliability and safety for commercial and consumer applications. In this paper, the bubble nucleation and evolution characteristics of subcooled flow boiling in high aspect ratio flat microchannels were experimentally studied with the aid of high-speed visualization and infrared temperature measurement at localized heat fluxes exceeding 1.5 kW/cm2. Dielectric liquid HFE-7100 was used at mass flow rates ranging from 11.1 to 178.2 kg/(m2·s). Results show that the explosive boiling process was accompanied by an approximately linear bubble growth rate of about 20 m/s at about the first half of growth stage, and the time duration for overall bubble growth was less than 200 μs. The bubble nucleation was suppressed dramatically on the smooth bottom surface of the channel even at a localized “hot spot” heat flux above 2.5 kW/cm2. The introduction of staggered short micro-pin fin arrays at the bottom wall significantly lowered the threshold heat flux and onset temperature of bubble nucleation and reinforced the bubble growth to larger sizes. It is attributed to the reduction in the temperature gradient of thermal boundary layer adjacent to the heated wall, and it would be further reduced by increasing the micro-pin fin denseness. Even at quite high superheats on smooth and micro-structured surfaces, the heterogeneous boiling mechanism was proved to still work.

Boiling and cavitation caused by transient heat transfer in superfluid helium-4

Superfluid helium-4 (He II) has been widely utilized as a coolant in various scientific and engineering applications due to its superior heat transfer capability. An important parameter required in the design of many He II based cooling systems is the peak heat flux ${q}^{*}$, which refers to the threshold heat flux above which boiling spontaneously occurs in He II. Past experimental and numerical studies showed that ${q}^{*}$ increases when the heating time ${t}_{h}$ is reduced, which leads to an intuitive expectation that very high ${q}^{*}$ may be achievable at sufficiently small ${t}_{h}$. Knowledge on how ${q}^{*}$ actually behaves at small ${t}_{h}$ is important for applications such as laser ablation in He II. Here we present a numerical study on the evolution of the thermodynamic state of the He II in front of a planar heater by solving the He II two-fluid equations of motion. For an applied heat flux, we determine the heating time beyond which the He II near the heater transits to the vapor phase. As such, a curve correlating ${q}^{*}$ and ${t}_{h}$ can be obtained, which nicely reproduces some relevant experimental data. Surprisingly, we find that there exists a critical peak heat flux ${q}_{c}^{*}$, above which boiling occurs nearly instantaneously regardless of ${t}_{h}$. We reveal that the boiling in this regime is essentially cavitation caused by the combined effects of the first-sound and the second-sound waves in He II. Based on this physical picture, an analytical model for ${q}_{c}^{*}$ is developed, which reproduces the simulated ${q}_{c}^{*}$ values at various He II bath temperatures and hydrostatic head pressures. This work represents a major progress in our understanding of transient heat transfer in He II.

The Role of Heat Flux in an Idealised Firebreak Built in Surface and Crown Fires

The disruptions to wildland fires, such as firebreaks, roads and rivers, can limit the spread of wildfire propagating through surface or crown fire. A large forest can be separated into different zones by carefully constructing firebreaks through modification of vegetation in firebreak regions. However, the wildland fire behaviour can be unpredictable due to the presence of either wind- or buoyancy-driven flow in the fire. In this study, we aim to test the efficacy of an idealised firebreak constructed by unburned vegetation. The physics-based large eddy simulation (LES) simulation is conducted using Wildland–urban interface Fire Dynamic Simulator (WFDS). We have carefully chosen different wind velocities with low to high values, 2.5~12.5 m/s, so the different fire behaviours can be studied. The behaviour of surface fire is studied by Australian grassland vegetation, while the crown fire is represented by placing cone-shaped trees with grass underneath. With varying velocity and vegetation, four values of firebreak widths (Lc), ranging from 5~20 m, is tested for successful break distance needed for the firebreak. For each failure or successful firebreak width, we have assessed the characteristics of fire intensity, mechanism of heat transfer, heat flux, and surface temperature. It was found that with the inclusion of forest trees, the heat release rate (HRR) increased substantially due to greater amount of fuel involved. The non-dimensional Byram’s convective number (NC) was calculated, which justifies simulated heat flux and fire characteristics. For each case, HRR, total heat fluxes, total preheat flux, total preheat radiation and convective heat flux, surface temperature and fire propagation mode are presented in the details. Some threshold heat flux was observed on the far side of the firebreak and further studies are needed to identify them conclusively.

Liquid film boiling on plain and structured tubular surfaces with and without hydrophobic coating

In this study, thin liquid film boiling heat transfer characteristics of R134a on a plain and two finned tubular surfaces were experimentally investigated. The dependence of heat transfer coefficient (HTC) on the composite effects of microstructure and hydrophobic coating was characterized. It is found that HTC increases monotonously with increasing heat flux until reaching a threshold heat flux (THF), beyond which the HTC starts to descend. The THF is highest for the plain surface and lowest for the boiling-enhanced surface due to the effect of reentrant cavities. The boiling-enhanced surface shows larger HTC under low heat fluxes (less than 60 kW m−2) compared with the condensation-enhanced counterpart, whereas the latter one is superior when the heat flux exceeds 60 kW m−2. Hydrophobic coating can substantially intensify the liquid film boiling heat transfer on boiling-enhanced finned surface with reentrant cavities. The intensification of heat transfer is more prominent in cases of high heat flux.

Threshold Heat-Flux Reduction by Near-Resonant Energy Transfer.

Near-resonant energy transfer to large-scale stable modes is shown to reduce transport above the linear critical gradient, contributing to the onset of transport at higher gradients. This is demonstrated for a threshold fluid theory of ion temperature gradient turbulence based on zonal-flow-catalyzed transfer. The heat flux is suppressed above the critical gradient by resonance in the triplet correlation time, a condition enforced by the wave numbers of the interaction of the unstable mode, zonal flow, and stable mode.

Predicting potential cambium damage and fire resistance in Pinus nigra Arn. ssp. salzmannii

Fire management can play a key role in ensuring stand maintenance in future scenarios of global change, particularly in Pinus nigra stands, which are known to be adapted to low-intensity surface fires through characteristics such as thick bark. In this study, laboratory tests were carried out to quantify cambium damage and fire resistance in P. nigra, by using a mass loss colorimeter device in a vertical configuration for the first time. In addition, low-intensity prescribed burning treatments were conducted in the field, and the field and laboratory data were compared. The following variables were used as proxy measures to assess cambium damage: time that temperature remained above 60 °C, heating rate and maximum absolute temperature in the inner bark area. The data were analysed using a Bayesian hierarchical approach (generalized linear mixed model). A threshold heat flux (25 kW m−2) for the time to ignition of bark was identified. A critical temperature of 60 °C was reached in the cambium during the combustion phase, after the flame was extinguished. The laboratory experiments showed, for the first time, the influence of flame residence time on the potential cambium damage. A bark thickness of 17 mm can be considered the threshold level for preventing critical temperatures being reached in Pinus nigra stands. The influence of bark thickness on protection against fire was confirmed, as was the importance of the coefficient of variation of bark thickness. The field results showed that flame characteristics (maximum temperature and residence time) were the most significant predictors of cambium damage. The combination of fire intensity and exposure time at low heat fluxes is more important than bark in determining cambium damage and may have important implications in the field of forest fuel management and in the ecology of pine forests.

Waves on the He-II Surface, Excited by a Heat Flux in the Bulk

The conditions for the occurrence of an instability generated on the free surface of superfluid He-II by a constant or variable heat flux Q in the He-II bulk have been experimentally studied. This instability is a quantum analog of the Kelvin–Helmholtz instability; it is accompanied by the occurrence of low-frequency oscillations on the surface and develops when the maximum velocity of the counterflow of the normal and superfluid components under the surface reaches the threshold value. It is shown that the large variance in the estimates (differing by a factor of 50–100) of the threshold heat flux Qthr (waves occur on the free He-II surface when the threshold is exceeded) in three series of experiments can be explained by different cross-sectional areas and numbers of channels in the lateral walls, through which the normal He-II component transfers heat from the immobile cell into the external helium bath (i.e., the significant difference in the boundary conditions). It is established that, switching on a heater, one can implement conditions in the liquid bulk that are sufficient for observing simultaneously the parametric Faraday instability and Kelvin–Helmholtz instability on the free He-II surface in a hermetically closed oscillating cell cooled from outside by superfluid He-II at a constant temperature.

Heat-to-mechanical energy conversion in graphene: Manifestation of Umklapp enhancement with strain

Conversion of heat-flux from a steady state temperature difference to mechanical vibration is demonstrated in graphene nanoribbons using direct non-equilibrium molecular dynamics. We observe that this effect is independent of the method of imposing the temperature gradient, heat flux, as well as imposed boundary conditions. We propose that simply dividing the nanoribbon in long and short sections using a partially immobilized area will lead to excitation of long-wavelength vibrations into the long section of the nanoribbon. This results in simpler architectures for heat-to-vibration converter devices based on graphene. Furthermore, we observe that applying tensile axial strain to nanoribbons facilitates vibrational instability by reducing the required threshold heat flux or the temperature gradient. Finally, we discuss the role played by Umklapp scattering for physical mechanisms behind these observations.

The effect of lateral thermal coupling between parallel microchannels on two-phase flow distribution

Evaporating flows in parallel channels occurring in a variety of industrial heat exchange processes can encounter non-uniform flow distribution between channels as a result of two-phase flow instabilities. Such flow maldistribution can have a negative impact on the performance, robustness and predictability of these systems. Two-phase flow modeling can assist in understanding the mechanistic behavior of this flow maldistribution, as well as determine parametric trends and identify safe operating conditions.The work described in this paper expands on prior two-phase flow distribution modeling efforts by including and assessing the effect of thermal conduction in the walls surrounding the parallel channels. This thermal conduction has a critical dampening effect on wall temperature gradients. In particular when a channel is significantly starved of flow rate and risks dryout, channel-to-channel thermal coupling can redistribute the heat load from the flow-starved channel to neighboring channels. The model is used to simulate the two-phase flow distribution in a system of two parallel channels driven by a constant flow rate pump. A comparison between thermally isolated and coupled channels indicates that thermally coupled channels are significantly less susceptible to maldistribution. Furthermore, a parametric study reveals that flow maldistribution is only possible in thermally coupled systems beyond a certain critical heat flux threshold. This threshold heat flux increases as the lateral wall conductance is increased, converging to a constant value in the limit of very high lateral conductance.

Structural impact of armor monoblock dimensions on the failure behavior of ITER-type divertor target components: Size matters

Plenty of high-heat-flux tests conducted on tungsten monoblock type divertor target mock-ups showed that the threshold heat flux density for cracking and fracture of tungsten armor seems to be related to the dimension of the monoblocks. Thus, quantitative assessment of such size effects is of practical importance for divertor target design. In this paper, a computational study about the thermal and structural impact of monoblock size on the plastic fatigue and fracture behavior of an ITER-type tungsten divertor target is reported. As dimensional parameters, the width and thickness of monoblock, the thickness of sacrificial armor, and the inner diameter of cooling tube were varied. Plastic fatigue lifetime was estimated for the loading surface of tungsten armor and the copper interlayer by use of a cyclic-plastic constitutive model. The driving force of brittle crack growth through the tungsten armor was assessed in terms of J-integral at the crack tip. Decrease of the monoblock width effectively reduced accumulation of plastic strain at the armor surface and the driving force of brittle cracking. Decrease of sacrificial armor thickness led to decrease of plastic deformation at the loading surface due to lower surface temperature, but the thermal and mechanical response of the copper interlayer was not affected by the variation of armor thickness. Monoblock with a smaller tube diameter but with the same armor thickness and shoulder thickness experienced lower fatigue load. The predicted trends were in line with the experimental observations.

Effect of viscosity on thermocapillary breakdown of a falling liquid film

Thermocapillary breakdown of a liquid film flowing due to gravity over a vertical plate with a heater of 150×150 mm is studied in a wide range of liquid properties (in particular, dynamic viscosity at the initial temperature varies from 0.91·10-3 to 16.9·10-3 Pa·s) and film Reynolds number (Re = 0.15-53.5). It is found that liquid viscosity has a significant effect on the threshold heat flux corresponding to film breakdown. To take into account the effect of liquid properties, the breakdown criterion traditionally used in literature was modified. This allowed successful generalization of all data obtained.

The effect of substrate wettability on the breakdown of a locally heated fluid film

The effect of the equilibrium contact angle of wetting on the dynamics of the dry patch propagation and on the critical heat flux upon the breakdown of a water film that is heated locally from the substrate side is studied experimentally. The equilibrium contact angle is varied from 27° ± 6° to 74° ± 9° (with no changes in the thermophysical properties of the system) through the use of different types of surface grinding. The studies are performed for three flow modes: (a) a fluid film that freely flows down along a substrate with an inclination of 5° to the horizon, (b) a film that moves along a horizontal substrate under the influence of hydrostatic pressure, and (c) a static film on a horizontal substrate. It is found that the substrate wettability has a significant effect on the dry patch propagation rate and its final size in all these cases, but has almost no effect on the threshold heat flux at which the breakdown of a film occurs.

TRIGGERING THE BOILING CRISIS: A STUDY OF THE DRY SPOT SPREADING MECHANISM

In this article we describe the boiling crisis (departure from nucleate boiling) as a transition between two regimes of bubble growth: "bubble departure" and "bubble spreading", which occur for small and large heat fluxes, respectively. The threshold heat flux is the critical heat flux (CHF). Our assumptions are based on existing experimental observations. The boiling crisis is assumed to be triggered by spreading of dry spots under individual bubbles that occurs due to the apparent contact angle growth, when the apparent contact angle attains 90°. Such a mechanism is studied via numerical simulation of the single bubble growth. The apparent contact angle is obtained from the hydrodynamic modeling of the vicinity of the triple contact line (microregion). The microregion model is based on the lubrication approximation extended to treat high interface slopes. The microregion model is compared to previous experimental and numerical data obtained for moderate heater superheating values. The macroregion is modeled within a simplified 2D quasistationary approach that neglects the fluid motion. The CHF obtained from such a formulation is calculated as a function of gravity level and the heater wetting properties. The results agree qualitatively with existing experimental data.

Fire safety distances for open pool fires

Fire accidents that carry huge loss with them have increased in the previous two decades than at any time in the history. Hence, there is a need for understanding the safety distances from different fires with different fuels. Fire safety distances are computed for different open pool fires. Diesel, gasoline and hexane are used as fuels for circular pool diameters of 0.5m, 0.7m and 1.0m. A large square pool fire of 4m×4m is also conducted with diesel as a fuel. All the prescribed distances in this study are purely based on the thermal analysis. IR camera is used to get the thermal images of pool fires and there by the irradiance at different locations is computed. The computed irradiance is presented with the threshold heat flux limits for human beings.

Investigation of the third heat transfer crisis on a vertical surface

The process of development of the third heat transfer crisis for vertical orientation of the heating surface was studied experimentally. Experiments were carried out with acetone under the conditions of saturation for the pressures in the working volume from 20 to 28 kPa. In all experiments, the third heat transfer crisis was preceded by propagation of evaporation front along the heating surface. The threshold values of heat flux densities, above which a stable vapor film is formed on the whole heating surface, are lower for vertical orientation of this heating surface than for the horizontal one. Data on the threshold heat flux densities and overheating before boiling-up were obtained. Above these values, formation of evaporation fronts was observed. The range of operation parameters corresponding to formation of the sites of unstable film boiling on the heating surface after boiling-up was determined.

Heat Flux Analysis of a Reacting Thermite Spray Impingent on a Substrate

Spray combustion from a thermite reaction is a new area of research relevant to localized energy generation applications, such as welding or cutting. In this study, we characterized the heat flux of combustion spray impinging on a target from a nozzle for three thermite mixtures. The reactions studied include aluminum (Al) with iron oxide (Fe2O3), Al with copper oxide (CuO), and Al with molybdenum oxide (MoO3). Several standoff distances (i.e., distance from the nozzle exit to the target) were analyzed. A fast response heat flux sensor was engineered for this purpose and is discussed in detail. Results correlated substrate damage to a threshold heat flux of 4550 W/cm2 for a fixed-nozzle configuration. Also, higher gas-generating thermites were shown to produce a widely dispersed spray and be less effective at imparting kinetic energy damage to a target. These results provide an understanding of the role of thermal and physical properties (i.e., such as heat of combustion, gas generation, and particle size...

Local heat transfer distribution and effect of instabilities during flow boiling in a silicon microchannel heat sink

Flow boiling of the perfluorinated dielectric fluid FC-77 in a silicon microchannel heat sink is investigated. The heat sink contains 60 parallel microchannels each of 100 μm width and 389 μm depth. Twenty-five evenly distributed temperature sensors in the substrate yield local heat transfer coefficients. The pressure drop across the channels is also measured. Experiments are conducted at five flow rates through the heat sink in the range of 20–80 ml/min with the inlet subcooling held at 26 K in all the tests. At each flow rate, the uniform heat input to the substrate is increased in steps so that the fluid experiences flow regimes from single-phase liquid flow to the occurrence of critical heat flux (CHF). In the upstream region of the channels, the flow develops from single-phase liquid flow at low heat fluxes to pulsating two-phase flow at high heat fluxes during flow instability that commences at a threshold heat flux in the range of 30.5–62.3 W/cm 2 depending on the flow rate. In the downstream region, progressive flow patterns from bubbly flow, slug flow, elongated bubbles or annular flow, alternating wispy-annular and churn flow, and wall dryout at highest heat fluxes are observed. As a result, the heat transfer coefficients in the downstream region experience substantial variations over the entire heat flux range, based on which five distinct boiling regimes are identified. In contrast, the heat transfer coefficient midway along the channels remains relatively constant over the heat flux range tested. Due to changes in flow patterns during flow instability, the heat transfer is enhanced both in the downstream region (prior to extended wall dryout) and in the upstream region. A previous study by the authors found no effect of instabilities during flow boiling in a heat sink with larger microchannels (each 300 μm wide and 389 μm deep); it appears therefore that the effect of instabilities on heat transfer is amplified in smaller-sized channels. While CHF increases with increasing flow rate, the pressure drop across the channels has only a minimal dependence on flow rate once boiling is initiated in the microchannels, and varies almost linearly with increasing heat flux.

Instability phenomena in a two-phase microchannel thermosyphon

The behavior of a two-phase thermosyphon, consisting of a microchannel evaporator plate and a condenser, is investigated to gain insight into the system limiting instability. A microchannel plate has been fabricated with 56 square channels that have a 1 × 1 mm cross section and a length of 115 mm. Experiments have been conducted for various condenser heights with the heat flux as the control variable. A step increase in heat flux is used to quantify the response of the system, including variations in mass flow rate, temperature, and pressure drop. Results show that small fluctuations about the steady state give rise to the instability for situations with a uniform heat load. A predictive model based on the momentum equation is introduced to estimate the onset of instability, and the threshold heat flux is predicted to within ±10% uncertainty.

An experimental modeling of gravity effect on rupture of a locally heated liquid film

This study deals with the formation of dry patches in a subcooled liquid film flowing over a locally heated plate at small positive and negative plate inclination angles with respect to the horizon. Prior to film rupture appreciable thermocapillary deformations of the film surface appear, growing with the heat flux. Upon reaching a threshold heat flux the film rupture occurs. By means of high speed imaging it is found that the process of rupture involves two stages: 1) abrupt film thinning down to a thin residual film on the heater; 2) rupture and dryout of the residual film. As the plate inclination angle is reduced the threshold heat flux required for film rupture weakly decreases, however when the angle becomes negative the threshold heat flux begins to rise dramatically, which is associated with an increase of the stabilizing hydrostatic effect due to the growth of the film thickness. The characteristic time of rupture decreases as the threshold heat flux increases. At nucleation of the dry patch the speed of contact line can be as high as 220 mm/s. The results obtained, apart from their intrinsic importance for ground-based applications, can also be of interest for microgravity research as a film flow with different relative contribution of inertia, hydrostatic and thermocapillary forces is considered.

Rupture of a subcooled liquid film falling down a heated grooved surface

This paper presents an experimental study of rupture of a subcooled water film falling down an 1 m long heated copper plate with longitudinal grooves of 0.5×0.15 mm2 cross sectional area and 2 mm spacing. It was found that the threshold heat flux at which an initial stable dry patch forms on the grooved surface is about two times higher than that on a smooth surface. Furthermore, the grooves prevent dry patches from spreading over the total heated surface thus essentially delaying the onset of the heat transfer crisis. The main governing parameters of the experiment and their respective values are: initial film temperature (20–95°C), heat flux (0–1.26 W/cm2) and volumetric flow rate (11.1–38.2 l/h) (Re=56.2–653.2).