Effects of spray characteristics on critical heat flux in subcooled water spray cooling

Effects of spray parameters (mean droplet size, droplet flux, and droplet velocity) on critical heat flux (CHF) were studied while these parameters were systematically varied. The effect of each parameter was studied while keeping the other two nearly constant. The mean droplet velocity (V) had the most dominant effect on CHF and the heat transfer coefficient at CHF (h_c), followed by the mean droplet flux (N). The Sauter mean diameter (d₃₂) did not appear to have an effect on CHF. By increasing V, CHF and h_c were increased. This trend was observed when all other spray parameters were kept within narrow ranges and even when relaxed to wider ranges, indicating the dominant effect of V. The effect of N, although not so much as V, was also found to be significant. Increasing N resulted in an increase in CHF and h_c when other parameters are kept in narrow ranges. A dilute spray with large droplet velocities appears to be more effective in increasing CHF than a denser spray with lower velocities for a given N. The mass flow rate was not a controlling parameter of CHF.     

Saturated flow boiling heat transfer and critical heat flux in small horizontal flattened tubes

This paper presents experimental results for flow boiling heat transfer coefficient and critical heat flux (CHF) in small flattened tubes. The tested flattened tubes have the same equivalent internal diameter of 2.2 mm, but different aspect height/width ratios (H/W) of ¼, ½, 2 and 4. The experimental data were compared against results for circular tubes using R134a and R245fa as working fluids at a nominal saturation temperature of 31 °C. For mass velocities higher than 200 kg/m²s, the flattened and circular tubes presented similar heat transfer coefficients. Such a behavior is related to the fact that stratification effects are negligible under conditions of higher mass velocities. Heat transfer correlations from the literature, usually developed using only circular-channel experimental data, predicted the flattened tube results for mass velocities higher than 200 kg/m²s with mean absolute error lower than 20% using the equivalent diameter to account for the geometry effect. Similarly, the critical heat flux results were found to be independent of the tube aspect ratio when the same equivalent length was kept. Equivalent length is a new parameter which takes into account the channel heat transfer area. The CHF correlations for round tubes predicted the flattened tube data relatively well when using the equivalent diameter and length. Furthermore, a new proposed CHF correlation predicted the present flattened tube data with a mean absolute error of 5%.     

Effect of channel geometry on critical heat flux for low pressure water

The main difficulty in interpreting critical heat flux (CHF) at low velocity and pressure conditions arises from the fact that the burnout phenomenon under such conditions is vulnerable to the effect of buoyancy and flow instabilities. This study is intended to provide some systematic understanding on CHF at low velocity and pressure conditions. Data obtained in the previous experiments for water in an annulus, rectangular ducts and a round tube are briefly reviewed and augmented in collaboration with existing data and correlations to extract more generic information. The effect of channel geometry on CHF is then discussed. The effect of channel geometry turned out to be remarkable at intermediate mass velocities. The difference in CHF at these mass velocities between a round tube and the other channel geometries was attributed mainly to the existence of an unheated wall which cause a non-uniform distribution of liquid film.     

Effects of high subcooling on two-phase spray cooling and critical heat flux

Experiments were performed with FC-77 using three full-cone spray nozzles to assess the influence of subcooling on spray performance and critical heat flux (CHF) from a 1.0 × 1.0 cm² test surface. The relatively high boiling point of FC-77 (97 °C at one atmosphere) enabled testing at relatively high levels of subcooling. Increasing the subcooling delayed the onset of boiling but decreased the slope of the nucleate boiling region of the spray boiling curve. The enhancement in CHF was relatively mild at low subcooling and more appreciable at high subcooling. CHF was enhanced by about a 100% when subcooling was increased from 22 to 70 °C, reaching values as high as 349 W/cm². The FC-77 data were combined with prior spray CHF data from several studies into a broad CHF database encompassing different nozzles, fluids, flow rates, spray orientations, and subcoolings. The entire CHF database was used to modify the effect of subcooling in a previous CHF correlation that was developed for relatively low subcoolings. The modified correlation shows excellent predictive capability.     

Derivation of mechanistic critical heat flux model for water based on flow instabilities

The critical heat flux (CHF) is a major parameter which determines the cooling performance and therefore the prediction of CHF with accuracy is of importance for the design and safety analysis of nuclear power plant (NPP). In this study, the mechanistic CHF model and correlation for water are derived based on flow excursion (or Ledinegg instability) criterion and the simplified two-phase homogeneous model. The relationship between CHF for the water and the principal parameters such as mass flux, heat of vaporization, heated length-to-diameter ratio, vapor-liquid density ratio and inlet subcooling is derived on the developed correlation. The developed CHF correlation predicts very well at the applicable ranges, 1 < P < 40 bar, 1,300 < G 27,00 kg/m²s and inlet quality is less than −0.1. The overall mean ratio of predicted to experimental CHF value is 0.988 with standard deviation of 0.046.     

Correlation of critical heat flux for flow boiling of water in mini-channels

In view of practical significance of a correlation of critical heat flux (CHF) in the aspects of engineering design and prediction, this study is aiming at evaluation of existing CHF correlations for flow boiling of water with available databases taken from small-diameter tubes, and then development of a new, simple CHF correlation. Available CHF databases in the literature for flow boiling of water in small-diameter tubes (0.33 < D_h < 6.22 mm) are collected, covering wide parametric ranges. Three correlations by Bowring, Katto and Shah are evaluated with the CHF data for saturated flow boiling, and three correlations by Inasaka–Nariai, Celata et al. and Hall–Mudawar evaluated with the CHF data for subcooled flow boiling. The Hall–Mudawar correlation and the Shah correlation seem to be the most reliable tools for CHF prediction in the subcooled and saturated flow boiling regions, respectively. In order to avoid the defect of predictive discontinuities often encountered when applying previous correlations, a simple, nondimensional, inlet conditions dependent CHF correlation for saturated flow boiling has been formulated. Its functional form is determined by the application of the artificial neural network and parametric trend analyses to the collected database. Superiority of this correlation has been verified by the database. The new correlation has a mean deviation of 16.8% for this collected databank, smallest among all tested correlations. Compared to many inordinately complex correlations, this new correlation consists only of a single equation.     

Effect of porous coating on critical heat flux

The critical hear flux (CHF) is studied experimentally in vertical tubes heated directly using power current (DC 2500 A, 15 V) and cooled with water at a low mass flow rate (0 ∼ 200 kg/m²s) and at a low pressure (1.0 ∼ 8.0 bar). We used a smooth tube and a tube with a porous coating layer sintered onto the inner surface. The results are compared with each other and with correlations by Katto [1] and Weber [2]. We determined enhancement of heat transfer as well as a negative effect of the porous coating below the expected value of critical heat flux.     

Correlation of high critical heat flux during flow boiling for water in a small tube at various subcooled conditions

High critical heat fluxes (CHFs) for subcooled boiling of water in a small tube were investigated experimentally. A platinum tube with an inner diameter of 1.0 mm and a length of 40.9 mm was used in the experiment. The upward flow velocity, the subcooling of water, and the outlet pressure of the experimental tube were varied to enable a parametric study of the CHFs. The flow velocity ranged from 9 to 13 m/s and the inlet subcooling ranged from 69 to 148 K. The boiling number decreased with increasing Weber number. The boiling number is also dependent on a non-dimensional parameter and the density ratio of liquid to vapor. A correlation for the high CHF of the small tube was obtained based on the experimental data. Finally, the high CHF correlation was evaluated using the CHF data obtained by other researchers.     

Natural convection on horizontal,inclined, and vertical plates with variable surface temperature or heat flux

An analysis is performed to study the flow and heat transfer characteristics of laminar free convection in boundary layer flows from horizontal, inclined, and vertical flat plates in which the wall temperature T_w(x) or the surface heat flux q_w(x) varies as the power of the axial coordinate in the form T_w(x) = T_∞ + axⁿorq_w = bx^m. The governing equations are first cast into a dimensionless form by a nonsimilar transformation and the resulting equations are then solved by a finite-difference scheme. Numerical results for fluids with Prandtl numbers of 0.7 and 7 are presented for three representative exponent values under each of the nonuniform surface heating conditions. It has been found that both the local wall shear stress and the local surface heat transfer rate increase as the angle of inclination from the horizontal γ increases or as the local Grashof number increases. An increase in the value of the exponent n or m enhances the surface heat transfer rate, but it causes a decrease in the wall shear stress. Correlation equations for the local and average Nusselt numbers are obtained for the special cases of uniform wall temperature (UWT) and uniform surface heat flux (UHF). Comparisons are also made of the local Nusselt numbers between the present results and available experimental data for the UHF case, and a good agreement is found to exist between the two.     

Non-heating simulation of pool-boiling critical heat flux

This paper presents a non-heating experimental method that simulates the critical heat flux (CHF) phenomenon in pool boiling. In the experiments, with providing controlled air flow through the holes on a plate submerged in a pool of water, the liquid sublayer (macrolayer) thickness and bubble departure frequency have been successfully measured by a conductance probe. The CHF is reasonably predicted by applying the measured parameters to a liquid macrolayer dryout model. The measured trends of the macrolayer thickness and bubble departure frequency with air mass flux are also consistent with the present understanding. As a result of this experimental study, it is expected that the non-heating method would be useful to investigate the various parametric effects on pool and flow boiling CHF, with avoiding the difficulty in heating and large electric power requirement even for complex geometries.     

Rewetting and maximum surface heat flux during quenching of hot surface by round water jet impingement

The transient cooling of hot stainless steel surface of 0.25 mm thickness is done with round water jet impingement. Initially, the surface was heated up to the temperature of 800 °C before the water was injected through straight tube type nozzle of 2.5 mm diameter and 250 mm length. During impingement cooling, the surface temperature was measured up to 12 mm radial distance away from the stagnation point. The jet exit to surface spacing, z/d, and jet Reynolds number, Re, varied in the range of 4–16 and 5000–24,000 respectively. The surface rewetting and transient heat flux of the test-surface was studied for these operating parameters.During impingement cooling process the initial rewetting occurred at stagnation region with the lowest wetting delay period. In fact, the rewetting temperature, rewetting velocity and the maximum heat flux reduced for extreme spatial location. However, the wetting delay increased significantly for the locations away from the stagnation point. The surface rewetting and transient heat flux were increased with the rise in jet Reynolds number, resulting in the enhancement in rewetting temperature, rewetting velocity and reduced wetting delay. The maximum heat flux was obtained for 4–6 mm radial location. The effect of jet exit to surface spacing on the rewetting parameters is found to be marginal. A correlation has been developed which predicted the maximum heat flux within an error band of ±10%.     

Boiling heat transfer and critical heat flux in liquid films falling on vertically-mounted heat sources

Boiling heat transfer measurements were obtained for an inert fluorocarbon (FC-72) liquid film injected over a vertical heated wall. Flow visualization revealed that vigorous boiling prior to burnout ruptured the liquid continuum, causing most of the film to separate from the heated wall leaving a thin liquid subfilm which maintained contact with the wall. The critical heat flux (CHF) was accompanied by dryout of the subfilm after total separation of the liquid near the upstream edge of the heater. A higher CHF was achieved by increasing the film velocity or by utilizing a shorter heater. Experimental data correlated favorably with predictions of a CHF model based on the Helmholtz instability and subfilm dryout.     

Studies on critical heat flux in flow boiling at near critical pressures

Experimental studies on critical heat flux (CHF) have been conducted in a uniformly heated vertical tube of 12.7 mm internal diameter and 3 m length at different reduced pressures ranging from 0.24 to 0.99 with R-134a as the working fluid. The onset of CHF was determined by the sudden rise in the wall temperature of the electrically heated tube. Experiments were performed over a wide range of parameters: mass flux values from 200 to 2000 kg/m² s, pressure from 10 to 39.7 bars and heat flux from 2 to 80 kW/m² and exit quality from 0.17 to 0.94. The results show considerably lower critical heat flux at high pressures. Well known CHF prediction methods, such as the look-up table and correlations of earlier workers show poor agreement at high pressures. A new correlation has been proposed to estimate the CHF in uniformly heated vertical tubes up to the critical pressure and over a wide range of parameters.     

Effect of tube bundles on nucleate boiling and critical heat flux

In order to elucidate boiling heat transfer characteristics for each tube and the critical heat flux (CHF) for tube bundles, an experimental investigation of pool and flow boiling of Freon-113 at 0.1 MPa was performed using two typical tube arrangements. A total of fifty heating tubes of 14 mm diameter, equipped with thermocouples and cartridge heaters, were arrayed at pitches of 18.2 and 21.0 mm to simulate both square in-line and equilateral staggered bundles. For the flow boiling tests the same bundles as were used in pool boiling were installed in a vertical rectangular channel, to which the fluid was supplied with an approach velocity varying from 0.022 to 0.22 m/s. It was found in this study that the boiling heat transfer coefficient of each tube in a bundle was higher than that for an isolated single tube in pool boiling. This enhancement increases for tubes at higher locations, but decreases as heat flux is increased. At heat fluxes exceeding certain values, the heat transfer coefficient becomes the same as that for an isolated tube. As the heat flux approaches the CHF, flow pulsations occurred in the pool boiling experiments although the heat transfer coefficient was invariant even under this situation. The approach velocity has an appreciable effect on heat transfer up to a certain level of heat flux. In this range of heat flux, the heat transfer coefficient exceeds the values observed for pool boiling. An additive method with two contributions, i.e., single phase convection and boiling, was used to predict the heat transfer coefficient for bundles. The predicted results showed reasonable agreement with the measured results. The critical heat flux in tube bundles tended to increase as more bubbles were rising through the tube clearance. © 1998 Scripta Technica, Heat Trans Jpn Res, 27(4): 312–325, 1998     

Correlation of critical heat flux and two-phase friction factor for subcooled convective boiling in structured surface microchannels

Critical heat flux (CHF) and pressure drop of subcooled flow boiling are measured for a microchannel heat sink containing 75 parallel 100 μm × 200 μm structured surface channels. The heated surface is made of a Cu metal sheet with/without 2 μm thickness diamond film. Tests and measurements are conducted with de-ionized water, de-ionized water +1 vol.% MCNT additive solution, and FC-72 fluids over a mass velocity range of 820–1600 kg/m² s, with inlet temperatures of 15(8.6)°C, 25(13.6)°C, 44(24.6)°C, and 64(36.6)°C for DI water (FC-72), and heat fluxes up to 600 W/cm². The CHF of subcooled flow boiling of the test fluids in the microchannels is measured parametrically. The two-phase pressure drop is also measured. Both CHF and the two-phase friction factor correlation for one-side heating with two other side-structured surface microchannels are proposed and developed in terms of the relevant parameters.     

Surface wettability effects on critical heat flux of boiling heat transfer using nanoparticle coatings

This study investigates the effects of surface wettability on pool boiling heat transfer. Nano-silica particle coatings were used to vary the wettability of the copper surface from superhydrophilic to superhydrophobic by modifying surface topography and chemistry. Experimental results show that critical heat flux (CHF) values are higher in the hydrophilic region. Conversely, CHF values are lower in the hydrophobic region. The experimental CHF data of the modified surface do not fit the classical models. Therefore, this study proposes a simple model to build the nexus between the surface wettability and the growth of bubbles on the heating surface.     

Nanofluids and critical heat flux,experimental and analytical study

In recent years, nanofluids have been attracting significant attention in the heat transfer research community. These fluids are obtained by suspending nanoparticles having sizes between 1 and 100 nm in regular fluids. It was found by several researchers that the thermal conductivity of these fluids can be significantly increased when compared to the same fluids without nanoparticles. Also, it was found that pool boiling critical heat flux increases in nanofluids. In this paper, our objective is to evaluate the impact of different nanoparticle characteristics including particle concentration, size and type on critical heat flux experimentally at saturated conditions. As a result, this work will document our experimental findings about pool boiling critical heat flux in different nanofluids. In addition, we will identify reasons behind the increase in the critical heat flux and present possible approaches for analytical modeling of critical heat flux in nanofluids at saturated conditions.     

Viscous dissipation effects on parallel plates with constant heat flux boundary conditions

This paper investigates basic analytical expressions for Nusselt number with the effect of viscous dissipation on the heat transfer between infinite fixed parallel plates, where the focus is on hydro-dynamically and thermally fully developed flow of a Newtonian fluid with constant properties, neglecting the axial heat conduction. Thermal boundary conditions considered are: both the plates kept at different constant heat fluxes, both the plates kept at equal constant heat fluxes, and one plate insulated. From the analysis, new expressions for Nusselt numbers have been found, as a function of various definitions of the Brinkman number.     

Wall heat flux partitioning during subcooled forced flow film boiling of water on a vertical surface

Subcooled flow film boiling experiments were conducted on a vertical flat plate, 30.5 cm in height, and 3.175 cm wide with forced convective upflow of subcooled water at atmospheric pressure. Data have been obtained for mass fluxes ranging from 0 to 700 kg/m²s, inlet subcoolings ranging from 0 to 25 °C and wall superheats ranging from 200 to 400 °C. Correlations for wall heat transfer coefficient and wall heat flux partitioning were developed as part of this work. These correlations derive their support from simultaneous measurements of the wall heat flux, fluid temperature profiles, liquid side heat flux and interfacial wave behavior during steady state flow film boiling. A new correlation for the film collapse temperature was also deduced by considering the limiting case of heat flux to the subcooled liquid being equal to the wall heat flux. The premise of this deduction is that film collapse under subcooled conditions occurs when there is no net vapor generation. These correlations have also been compared with the data and correlations available in the literature.