Onset Of Nucleate Boiling Research Articles

Onset of Nucleate Boiling Model for Rectangular Upward Narrow Channel: CFD Based Approach

Despite that mechanistic and accurate correlations predicting the Onset of Nucleate Boiling (ONB) for pool boiling are widely presented in the literature, models for forced convective boiling remain few. These models do not provide the desired quality, principally because they do not consider important features of convective boiling. In this work, numerical investigations of the ONB for water boiling flow at atmospheric pressure upward a narrow rectangular channel (3 mm × 100 mm × 400 mm) are carried out based on Computational Fluid Dynamics (CFD) simulations. The predictions of the CFD calculations are validated with the available experimental data. A new ONB model incorporating the convective boiling features is developed and proposed. This model is derived based on several CFD simulation data, covering wide operating conditions. The flow Reynolds number ranges from 959 to 13500, inlet subcooling from 2.5 to 30 K and applied heat flux from 5 to 90 kW/m2. The new model predictions have a standard deviation of 2.7% where its performance is better than ±0.3 K when compared with additional simulation data that are provided for validation.

Minimal heating rate for isobaric nucleation at the spinodal in liquids

A thermodynamic model is developed that resolves the minimal heating rate required for rapid isobaric nucleation at the spinodal limit for both homogenous and heterogenous nucleaiton. For heterogeneous nucleation, a semi-empirical correlation for the heterogeneity factor is developed.The question of how fast should isobaric heating be in order for Onset of Nucleate Boiling (ONB) to occur at spinodal conditions can be answered currently for 1 atmosphere. For water at 1 atmosphere, it was found that the minimal heating rate for heterogeneous nucleation is about 109 K/s, which is consistent with the available experimental results. The proposed model, however, allows to answer this question for a wide rage of pressure values.The effects of pressure, initial temperature, transition time into the metastable zone and heterogeneity factor are explored.It was found that the minimal heating rate decreases with the decrease of the degree of superheating and the increase in the degree of subcooling. It follows that the minimal heating rate required for reaching the spinodal decreases with the increase of pressure. For heterogeneous nucleation as a result of wall heating, it decreases with the decrease in the initial temperature.An explicit, analytical function is offered to correlate the minimal heating rate for water to the degree of subcooling at various pressure values, up to Pr∼0.5.

Improved flow boiling performance in high-aspect-ratio interconnected microchannels

High aspect ratio microchannels have the advantage of dissipating more heat per base area. However, flow boiling instability of bubble expansion towards upstream and downstream is more easily triggered as soon as boiling starts due to the narrower channel width of high aspect ratio microchannel. To suppress flow boiling instability, in this study, the flow boiling heat transfer performance is experimentally investigated in the high-aspect-ratio (aspect ratio of 2.5) interconnected microchannels (IM) with micro connection slots. Two slots of 20 μm and 40 μm in width, denoted as IM20 and IM40, respectively, are considered. Experiments are conducted at mass fluxes of 446-963 kg/m2•s and effective heat fluxes of 36.2-427.9 W/cm2, using deionized water as the working fluid. The outlet of the channel is maintained at atmospheric pressure with a saturation temperature of 100 °C. Results show that obviously earlier onset of nucleate boiling (ONB) in IM is demonstrated, compared to that in plain-wall microchannel (PM). Meantime, the nucleation bubble is easily captured in IM, whereas almost no bubbly flow is observed in PM. More importantly, significantly stable flow boiling from the incipience of boiling to near the critical heat flux (CHF) point is found in IM. It is observed that regular liquid film redevelopment and annular flow appear alternately among parallel microchannels in IM. By contrast, due to flow maldistribution of PM, various irregular flow patterns appear simultaneously among parallel microchannels. These micro connection slots promote nucleate boiling and fluid mixing of neighboring channels. Eventually, remarkable heat transfer enhancement is achieved in IM without compromising the two-phase pressure drop. The enhanced heat transfer coefficient and critical heat flux of IM are respectively around 25.7%-86.9% and 13.9%-59.1% over the entire range of tested cases. In addition, owing to enhanced fluid mixing and more liquid maintaining in the larger slot, IM40 shows more heat dissipation than IM20. This study demonstrates that the microchannel heat sink with the interconnected configuration with tens of micro-slots can be an effective approach for suppressing flow boiling instability and enhancing flow boiling heat transfer performance.

Statistical method for determining the onset of nucleate boiling under forced and natural convections in a rectangular channel

A statistical method based on temperature change with heat flux in a single-phase flow is proposed to determine the onset of nucleate boiling (ONB) incipience. Experiments under forced and natural convections were performed with different inlet temperatures. ONB wall temperatures determined by the proposed method were compared with that determined by the intersection method. The difference between ONB wall temperatures obtained by the proposed and intersection methods under forced convection was insignificant; however, the difference was significant under natural convection. In addition, ONB wall temperatures for forced and natural convections were compared for predictions by existing ONB correlations, namely, Bergles and Rohsenow (1964), Jens and Lottes (1951), and Thom et al. (1965). The results showed that the Jens-Lottes correlation overestimated the ONB incipience, whereas the Bergles-Rohsenow correlation underestimated the ONB incipience. However, the existing correlations are still applicable for predicting ONB wall temperatures at extremely low flow rates.

Effects of Heat Flux, Mass Flux, Inlet Fluid Subcooling, and Channel Orientation on Subcooled Flow Boiling in a High-Aspect-Ratio Rectangular Microchannel

Abstract An experimental investigation of subcooled flow boiling in a high-aspect-ratio, one-sided heating, silicon-based rectangular microchannel was conducted utilizing de-ionized water as the working fluid. The microchannel was 5.01 mm wide and 0.52 mm high, having a hydraulic diameter of 0.94 mm and an aspect ratio (AR) of 10. The heat flux, mass flux, and inlet fluid subcooling were in the ranges of 0–30 W/cm2, 200–500 kg/m2 s, and 5–20 °C, respectively, while the orientations were vertical and horizontal. Parametric study on heat transfer characteristics including the onset of nucleate boiling (ONB), heat transfer coefficient (HTC), and critical heat flux (CHF) was carried out combined with flow visualization. Significant appearance of ONB without boiling hysteresis was observed in the boiling curve, accompanied with bubble nucleation. Nucleate boiling occurred first near the exit, where the HTC increased more sharply, while easier bubble nucleation was found near the sides. Unique time-dependent flow pattern consisting of isolated bubbly flow, elongated bubbly flow, partial dry-out, and rewetting process was observed. More nucleation sites were activated at higher heat flux, while higher initial heat flux and wall superheat for ONB as well as higher CHF value were obtained at higher mass flux and inlet subcooling. Compared to the vertical channel, higher wall temperature and pressure drop with larger oscillation amplitudes were found for the horizontal counterpart, where the merged bubbles agglomerated in the heating section, resulting in earlier dry-out which deteriorated heat transfer.

Experimental study of boiling flow in a vertical heated annulus with local two-phase measurements and visualization

Understanding the phenomena and predicting the behavior of two-phase flow are critical for safe design and operation of boiling systems. Local two-phase flow-boiling data were collected in an internally heated vertical annulus facility with a heated length of 3.0 m, an inner diameter of 19.05 mm, and an outer diameter of 38.10 mm. The data were collected with four-sensor probes which measure the void fraction, interfacial area concentration, and gas interface velocity at eleven radial locations over five axial locations within the boiling channel. In addition, the flow structure was captured with a high-speed camera at five viewports in between each of the instrumentation ports, and the Onset of Nucleate Boiling (ONB) and Point of Net Vapor Generation (PNVG) were identified for each of the conditions studied. The flow visualization shows the development of bubbly and slug flows within the channel which is accelerated for the low-pressure conditions. The local two-phase measurements show substantial void generation for the low-pressure conditions matched with an acceleration of the flow through the gas interface velocity. The interfacial area concentration is dominated by smaller group-1 bubbles and shows a decrease in total interfacial area concentration when larger group-2 bubbles dominate. This study provides detailed, accurate data for both the validation of multiphase computational fluid dynamics and system-analysis codes as well as the development of constitutive models in boiling flow.

Experimental Study of the Onset of Nucleate Boiling in Vertical Narrow Channels

Heat transfer mechanism in flow boiling through vertical narrow channels differs from that in larger circular tubes with diameters more than 10mm due to the increased surface forces. Basing on the experimental data, the main factors which have influence on heat flux of the onset of nucleate boiling (ONB), such as mass flux, system pressure and dimension of the narrow channel, are analyzed in this paper. The empirical correlation which used to calculate heat flux of the onset of nucleate boiling in vertical narrow annular channels in similar conditions is given here. The calculation results of this correlation are compared with the experimental data and agree well with it while the deviation is less than ±15%. And it’s concluded that narrow channels could enhance flow boiling heat transfer.

Pool boiling heat transfer on silicon chips with non-uniform micro-pillars

This paper investigated the pool boiling heat transfer performance of non-uniform micro-pillar surfaces. Three kinds of uniform micro-pillar surfaces (named U-S, U-M and U-L) and two kinds of non-uniform micro-pillar surfaces (named R-SML and R-SM) were fabricated to be tested, and a smooth surface (named SS) was prepared as a comparison. Experiments were performed in FC-72 at the subcoolings of 0 K and 25 K in both horizontal and vertical orientations. The results showed that the heat transfer coefficient (HTC) and critical heat flux (CHF) of surfaces with micro-pillars have been greatly enhanced compared to those of the smooth surface. Active nucleation site radius was discussed to explain the advanced ONB (onset of nucleate boiling) of U-M, R-SML and R-SM, then SEM images were token to prove this theory. Boiling images of 6 W/cm2 at 25 K subcooling were token by a high-speed camera to analyze the HTC of different chips. Surface area enhancement ratio and wickability were used for the explanation of CHF comparison. The results showed that surface area enhancement ratio and wickability are not the decisive factors that influences CHF for FC-72, and the uniformity of microstructures should be considered when predicting the CHF. Non-uniform micro-pillars with small surface area enhancement ratio and wickability can have larger CHF than that of uniform micro-pillars with large surface area enhancement ratio and wickability.

Experimental study of heat transfer at the transition regime between the natural convection and nucleate boiling: Influence of the heated wall tilt angle on the onset of nucleate boiling (ONB) and natural convection (ONC)

The works carried out in this study aimed to get a better understanding of the heat transfer in natural convection and nucleate boiling as well as the transition between these two regimes. An experimental test set up was built to generate the liquid boiling on the wall using a boiling-meter. This sensor was developed in order to investigate the heat transfer and its dependence with time in well controlled conditions. Temperature, pressure and heat flux required for the measurements were implemented. Experiments were focused on determining the characteristic curves of heat transfer in the natural convection regime and the nucleate boiling regime. The influence of the orientation of the wall relative to the gravity on the heat transfer is investigated. Characteristic heat transfer curves were obtained in several operating conditions and wall orientation with regard to gravity.At the transition regime between natural convection and nucleate boiling regimes, it was highlighted a competition between these two regimes. It was found, that the transferred heat flux differs depending on the orientation of the wall and the degree of liquid superheat in the nucleate boiling regime. In the natural convection regime, the measured heat fluxes exhibited a slight variation with the tilt angle of the wall. While in the nucleate boiling regime, strong variations were observed on the onset of nucleate boiling as well as for the heat flux. At very low heat flux, the heat transfer increases with the tilt angle while it decreases for moderate heat flux. The onset of nucleate boiling (ONB) and the onset of dominant natural convection (ONC) were found to decrease as a function of the tilt angle of the wall with different variation laws. The wall superheats values for the ONB and ONC are getting closer when the tilt angle tends to 180°.

Power upgrading of WWR-S research reactor using plate-type fuel elements part I: Steady-state thermal-hydraulic analysis (forced convection cooling mode)

The design of a nuclear reactor core requires basic thermal-hydraulic information concerning the heat transfer regime at which onset of nucleate boiling (ONB) will occur, the pressure drop and flow rate through the reactor core, the temperature and power distributions in the reactor core, the departure from nucleate boiling (DNB), the condition for onset of flow instability (OFI), in addition to, the critical velocity beyond which the fuel elements will collapse. These values depend on coolant velocity, fuel element geometry, inlet temperature, flow direction and water column above the top of the reactor core. Enough safety margins to ONB, DNB and OFI must-emphasized. A heat transfer package is used for calculating convection heat transfer coefficient in single phase turbulent, transition and laminar regimes. The main objective of this paper is to study the possibility of power upgrading of WWR-S research reactor from 2 to 10 MWth. This study presents a one-dimensional mathematical model (axial direction) for steady-state thermal-hydraulic design and analysis of the upgraded WWR-S reactor in which two types of plate fuel elements are employed. FOR-CONV computer program is developed for the needs of the power upgrading of WWR-S reactor up to 10 MWth.

Estimation of the Possibility of Using the Classical Theory of Nucleation in the Calculation of the Limiting Superheat for a Metastable Liquid in the Region of Onset of Nucleate Boiling

Estimation of the Possibility of Using the Classical Theory of Nucleation in the Calculation of the Limiting Superheat for a Metastable Liquid in the Region of Onset of Nucleate Boiling

Experimental investigation of flow boiling characteristics in microchannel with triangular cavities and rectangular fins

Phase change heat transfer in micro heat sink was an effective method to solve the thermal issues of high heat flux micro devices. The geometric structure modification of the micro heat sink could enhance its heat transfer performance obviously. In this paper, a microchannel with triangular cavities and rectangular fins (Tri.C-Rec.F) was fabricated for the purpose of flow boiling improvement. Flow boiling experiments were carried out using pure acetone with a fixed inlet temperature of 29 °C at four mass fluxes 83 kg/(m2⋅s), 147 kg/(m2⋅s), 324 kg/(m2⋅s) and 442 kg/(m2⋅s), effective heat flux ranging from 0 to 101 W/cm2. The flow boiling characteristics of the microchannel Tri.C-Rec.F were studied and compared with those of the conventional rectangular microchannel (R). The physics behind the flow boiling process and the heat transfer enhancement mechanisms of the micro heat sink were explored. Experimental results indicated that the microchannel Tri.C-Rec.F showed significant enhanced heat transfer, reduced onset of nucleate boiling (ONB), and delayed critical heat flux (CHF) compared to the microchannel R because of the increased bubble nucleation and the particular flow boiling phenomena. Moreover, the modified microchannel improved the flow boiling stability distinctly for high mass fluxes. The continuously developing liquid film could enhance the liquid film evaporation, maintain the liquid supplement and prevent partial dryout effectively. The flow disturbance effect of the micro structures and the bubble breaking effect of the micro fins promoted bubble departure and mitigated the flow reversal. The microchannel Tri.C-Rec.F presented a remarkable higher heat transfer coefficient than microchannel R with a maximum increment of 300% and 51.6% at G = =83 and 442 kg/(m2⋅s), respectively. The acceleration, disturbance and separation effects of the micro fins increased the pressure drop, which could be improved by further structural optimization.

Flow boiling instability characteristics in microchannels with porous-wall

In this work, a porous-wall microchannel heat sink was designed and fabricated, and the wall regions were etched with the micro pin fin arrangement instead of solid walls. The porous-wall can be divided into three regions: the densely/intermediate densely/sparely pin fin region with the fin gap of 5 μm/7.5 μm/10 μm, respectively. Flow boiling tests in porous-wall microchannels were carried out at mass fluxes of 250–510 kg/(m2·s) and heat fluxes of 120–720 kW/m2. The effects of the porous-wall on suppressing the flow instabilities and manipulating the boiling flow were studied, and different boiling flow behaviors of pressure drops and temperatures as well as corresponding flow patterns were measured and observed. The experimental results showed that: (1) With the porous-wall inter-connect effect, the premature ONB (Onset of Nucleate Boiling) can be occurred, the nucleate boiling flow in whole channel can be triggered in less than 2 ms, the boiling flow instability can be suppressed, and the duration of two-phase flow can be prolonged; (2) The boiling instabilities in porous-wall microchannels were classified into four flow modes, and the temporal behaviors of temperatures and pressure drops corresponding to each flow instability mode were observed and analyzed. (3) By PSD (Power Spectral Density) combined with WT (Wavelet Transform) analysis method, the coupled wall temperature oscillations in each instability type were decoupled into different instability sub-types at different amplitudes and timescales. Moreover, the analysis of ms-timescale flow instability induced by the flow pattern transition helps to better understanding the dominant influence of the porous-wall on the resulting temperature oscillations.

Transitional cores and fuel cycle analyses in support of MIT reactor low enriched uranium fuel conversion

The Massachusetts Institute of Technology Reactor (MITR) is a research reactor in Cambridge, Massachusetts designed primarily for experiments using neutron beam and in-core irradiation facilities. At 6 MW, it delivers neutron flux and energy spectrum comparable to power light water reactors (LWRs) in a compact core using highly enriched uranium (HEU) fuel. In the framework of non-proliferation policy, research and test reactors have started a program to convert HEU fuel to low enriched uranium (LEU) fuel. A new type of LEU fuel based on a high density alloy of uranium and molybdenum (U-10Mo) is expected to allow conversion of U.S. high performance research reactors (USHPRRs) like the MITR. The Preliminary Safety Analysis Report (PSAR) for the MITR LEU conversion has been submitted and currently under review by the U.S. Nuclear Regulatory Commission (NRC). A transition core plan, from 22 fresh LEU fuel elements (i.e., beginning-of-life) gradually to 24 of them arranged in an equilibrium configuration, is expected to serve as an appendix chapter in the final SAR. The current study presents the fuel cycle development, which eventually leads to the transition core plan. The results confirm the equilibrium state, where both the shim bank movement (i.e., core reactivity) as well as the content of fissile materials stabilize, can be achieved by fixed-pattern fuel management. Fission density has been evaluated for a number of fully discharged LEU fuel elements, using a best-estimate approach. The fuel cycle calculations also generate power profiles at each core state. A steady-state thermal-hydraulic safety analysis has thus been performed, where onset of nucleate boiling (ONB) is considered as the safety criterion. The results confirm significant margin to ONB at all analyzed transition and equilibrium fuel cycle states.

Experimental study on flow and heat transfer characteristics at onset of nucleate boiling in micro pin fin heat sinks

The micro pin fin heat sink has great application potential in the heat dissipation of high-power LED chips, phased array radars and semiconductor lasers. The onset of nucleate boiling (ONB) is a critical limit in designing heat dissipation systems. A simultaneous visualization and measurement study is conducted to investigate the ONB phenomenon of deionized water in circular, diamond, and oval micro pin fin heat sinks. Experiments are conducted under inlet subcooling of 29.6–49.5 °C, mass flux of 147.4–355.4 kg m−2 s−1 and heat flux of 4.8–55.8 W cm−2. Visualization result indicates that the ONB is always initiated at the surrounding region of a micro pin fin. The effects of additional nucleation sites caused by a micro pin fin and the temperature distribution of the surrounding region captured by an infrared camera can be used to explain the occurrence of the first bubbles. In this study, the effects of inlet subcooling, mass flux, and the cross-section shape of the micro rib on the ONB characteristics are investigated. Results show that the heat flux and wall superheat needed for the ONB increase as the mass flux and inlet subcooling increase. Moreover, the heat flux required for the ONB in the diamond and oval micro pin fin heat sinks is higher than that in the circular one. On the basis of the experimental data, the correlation of heat flux at the ONB in the micro pin fin heat sink is established, with 85.7% of the data falling within an error band of ±15%.

Numerical simulation of flow boiling in an orthogonally rotating duct

Numerical predictions of flow boiling in a square tube rotating in vertical direction at a uniform rotating speed are presented. There-dimensional physical model and numerical model were established for simplified problem. Realizable k−e (RKE) turbulent model was employed to study vapor–liquid two-phase turbulent flow. Heat and mass transfer between vapor and liquid were solved based on volume of fluid multiphase flow model combined with user-defined function. The transient results show that wall superheat at onset of nucleate boiling (ONB) condition in rotating tube is obviously higher than that in stable tube. Based on Jens–Lottes formula of Bowring model, empirical formulas for wall superheat at ONB condition were modified with consideration of Rossby number. As the rotation speed increased, location of ONB shifts to the tube exit. Flow pattern in tube is determined by coupling effect of heating and rotation. Stability of flow pattern is gradually weakened during the increase in rotation speed. The numerical results are in fair agreement with two sets of experimental data for same physical model and working conditions.

Markedly enhanced pool boiling heat transfer performance on maicropaorous copper surfaces fabricated utilizing a facile wire cutting process

The superior boiling surfaces obtained by facile, efficient and economical approaches are highly desirable in large-scale industrial applications. Here, a microporous copper surface for enhanced boiling was readily and rapidly prepared by utilizing a single-step and low-cost wire cutting electric discharge machining process. The saturated pool boiling tests on the microporous and plain copper surfaces were conducted at atmospheric pressure using distilled water as working fluid. The significant boiling performance enhancements are demonstrated on the microporous copper surface. It achieves a heat transfer coefficient (HTC) of 16.7 W cm−2 K−1, a critical heat flux (CHF) of 185.7 W cm−2 and a onset of nucleate boiling (ONB) at 4.4 K, indicating a 198% higher HTC, a 61% higher CHF, as well as a 47% lower ONB compared with the plain surface. The enhanced boiling mechanism is explained based on the surface characteristics. The formation of numerous microscale cavities facilitates bubble nucleation, departure for reducing ONB and increasing HTC, and improves surface wettability for delaying CHF. The visualization investigations of bubble behaviors were also conducted for further verifying our analysis. The results show the microporous copper surface exhibits a larger nucleation density, departure frequency, and a smaller departure diameter of bubble.

Bubble dynamic parameters during subcooled flow boiling under uniform and non-uniform transverse heat distribution

Bubble behaviors and their interactions with each other affect the two-phase flow characteristics. In this study, experiments are conducted to investigate the bubble characteristics during subcooled flow boiling under uniform and non-uniform transverse heat flux distribution. The non-uniform heat flux distribution creates non-uniform bubble characteristics on the heated surface as opposed to under uniform heating. The working fluid is demineralized water that flows through a narrow rectangular channel heated from one side. The experimental loop used in the study operates at low pressure. A wide range of experimental operating conditions, such as inlet temperature (35–65 °C), thermal power (500–6250 W), and mass flow rates (0.03–0.13 kg/s), are applied to the upward flow channel. The bubble behaviors are visualized using a high-speed camera (2200 fps) at a resolution of 512 × 512 pixels. The results indicate that the bubbles exhibit different departure diameters, nucleation site density distributions, and bubble departure frequencies. In the uniform case, bubbles are generated uniformly across the whole transverse direction of the heated surface. In the non-uniform case, more bubbles are generated where the heat flux is concentrated, which disturbs the flow velocity profile in the transverse direction. The differences in bubble generation in the transverse direction result in differences in the two-phase flow instability through the heated channel. As a result, new empirical correlations are proposed based on the experimental results to estimate the bubble departure diameter, nucleation site density, and bubble departure frequency. The correlations are applicable for both heated surface conditions, uniform and non-uniform, under low-pressure conditions. CFD analysis using ANSYS FLUENT incorporates the RPI wall boiling model is conducted to validate the empirical correlations. Comparison of the CFD calculations with the experimental data for void fraction, wall temperature, and bulk temperature show good agreement. The simulation results show an accurate prediction of the ONB (Onset of Nucleate Boiling) and OFI (Onset of Flow Instability).

Thermal-hydraulic analyses of MIT reactor LEU transition cycles

The Massachusetts Institute of Technology Reactor (MITR) is a research reactor in Cambridge, Massachusetts designed primarily for experiments using neutron beam and in-core irradiation facilities. In the framework of non-proliferation policy, research and test reactors have started a program to convert highly enriched uranium (HEU) fuel to low-enriched uranium (LEU) fuel. A new type of LEU fuel based on a high-density alloy of uranium and molybdenum (U-10Mo) is expected to allow conversion. A transition core plan, from 22 fresh LEU fuel elements to 24 in an equilibrium state, is proposed. This study evaluates the thermal-hydraulic safety margins of each transition cycle and state (i.e. beginning, middle and end). The STAT7 and RELAP5 codes are used in the study. STAT7 provides an integrated platform for statistical propagation of uncertainties in determining steady-state thermal-hydraulic margins. A RELAP5 model was created to verify MITR's STAT7 model, and provide transient analyses.The steady-state analyses determined that, during the entire transition from a fresh to equilibrium core, the Limiting System Safety Settings (LSSS) has a 10.2% margin to the minimum power at which onset of nucleate boiling (ONB) is precluded at 3σ confidence. The loss of flow (LOF) transient, which is the most limiting anticipated transient for the MITR, was simulated. With a conservative initial state of the reactor (operating at μ+3σ power level and LSSS mass flow rate), the maximum calculated centerline fuel temperature was 115 °C, which is a significantly lower value than the blistering limit of 350 °C. The cladding wall temperatures did not exceed the ONB temperature throughout the transient. Therefore, during LOF, no nucleate boiling is expected, precluding any critical heat flux trigger. This study indicates that the proposed LEU transition core specifications have significant margin to thermal-hydraulic limits, during steady-state operation and the LOF transient.

Experimental investigation of flow boiling heat transfer performance in zigzag microchannel heat sink for electronic cooling devices

This study proposes an improved microchannel heat sink with offset zigzag cavities to enhance boiling performance. Flow and heat transfer characteristics are experimentally investigated via a microscale boiling heat transfer visualization system using acetone as the working fluid. At mass fluxes of 285, 402, and 684 kg/(m2·s) and heat fluxes of 1.31–80.26 W/cm2, the effects of heat and mass fluxes on flow pattern, heat transfer, wall temperature, and pressure drop of zigzag microchannel heat sink are analyzed and compared with conventional rectangle microchannel heat sink. Results show that the fluctuating amplitude first increases and then decreases with the increment of heat fluxes, thereby causing a quasi-steady and unstable boiling with small fluctuations. Instability becomes increasingly severe for the nucleate boiling area moves toward the inlet and flow reversal intensity increases with heat fluxes under low heat flux. With heat flux increased furtherly. The proportion of steam in the channel increases to reduce fluctuations. Compared with the rectangular microchannel, the zigzag microchannel evidently enhances heat transfer characteristic with lower wall temperature at the onset of nucleate boiling (ONB), higher heat transfer coefficient (HTC) and critical heat flux (CHF) and improves boiling stability by suppressing flow reversal and reducing two-phase pressure drop. The mechanism of zigzag microchannel enhancement boiling performance can be attributed to (1) the improved disturbance of the working fluid, which enhances forced convective boiling heat transfer and mitigates bubble coalescence and flow reversal strength; (2) the continuously developing liquid film and improved wettability maintain a stable liquid film evaporation and delay partial dry out. Increase in mass flux can improve CHF and reduce pressure drop albeit with high wall temperature of ONB. The local HTC is enhanced, except for the low heat fluxes, because the bubbles are condensed by liquid and cannot grow to absorb heat at low heat fluxes.