Critical Heat Flux Regime Research Articles

Critical heat flux correlations for tube and annulus geometries under inclination and rolling conditions

In floating nuclear reactors, critical heat flux (CHF) prediction is crucial because the CHF can be influenced under heaving, rolling, and inclination conditions, unlike in land-based reactors. Hence, the existing land-based empirical correlations of the CHF can be invalid under some conditions in marine reactors. Therefore, empirical correlations of the flow boiling CHF were developed in this study to predict the CHF under inclination and rolling motions in tubes and annuli. Available experimental CHF data (616 points) were collected including our previous experimental data (405 points) and their trends under marine conditions were analyzed. The CHF variation was expressed using two types of modified Froude numbers depending on the CHF regime—departure from nucleate boiling and dryout. These parameters were used in correlations for the CHF under inclination. CHF correlations under rolling conditions were developed. Rolling was found to attenuate the CHF variation through the inclination. This observation was reflected in the newly developed rolling CHF correlation. The proposed correlations adequately described the existing CHF experimental data under inclination and rolling within an uncertainty of 9.5%. The developed CHF correlations can be used for the safety analysis and reactor core thermal–hydraulic design of marine reactors.

Analytical investigation on rod bundle CHF-regime criterion based on dimensionless groups

The accurate prediction of rod bundle critical heat flux (CHF) is an important task for thermal-hydraulic design of pressurized water reactor (PWR) core. Under normal and transient conditions of PWR, based on the two-phase flow pattern, there are different CHF regimes. In order to correctly distinguish the CHF regimes in rod bundle, it is necessary to analytical investigate rod bundle CHF-regime criterion. In this study, a general dimensionless rod bundle CHF correlation is presented based on dimensional analysis method. The real vapor quality, Katto number and boiling length are introduced to establish the basic form of the dimensionless CHF correlation. According to the deep analysis of the distribution trend of typical CHF experimental data and the CHF characteristics of different CHF regimes, the CHF in rod bundle are separated into DO, DNB and HP-DNB regimes. Concise dimensionless CHF correlations corresponding to each regime are developed, and the dimensionless equations between the boundaries of each regime are established. Also, a clearly defined map of characteristic regimes of CHF in rod bundle is determined. This study proposes a classification criterion for rod bundle for rod bundle CHF regimes, and it is very important for the upcoming development of rod bundle CHF mechanistic model.

Heat transfer characteristics of urea-water spray impingement on hot surfaces

This study presents an investigation of the heat transfer characteristics of the urea-water spray (UWS) impingement on a stainless steel plate under typical diesel exhaust flow conditions. The rear side temperature of the spray-impinged plate has been measured by infrared thermography with high temporal and spatial resolution. The spray impinged side temperature and heat flux distributions have been computed by solving the 3D inverse heat conduction in the plate with the sequential function specification method.Measurements show that the instantaneous plate temperature determines the heat transferred during the spray impingement. Based on the plate temperature, different regimes (film boiling, transition boiling and nucleate boiling) have been identified. At high plate temperatures, film boiling and Leidenfrost effect are prevailing and the heat transferred from the plate to the liquid is low. With decreasing plate temperatures, the critical heat flux regime is approached and the heat transferred increases substantially. At lower plate temperatures, nucleate boiling occurs limiting the heat transferred to low values. The critical heat flux and temperature found for UWS are reported and in good agreement with the trend in previous studies for water.

Flattening of boiling curves at post-CHF regime in the presence of localized electrostatic fields

We studied two-phase cooling heat transfer enhancement from a heated disk through falling jets of dielectric fluid, HFE7100, in the presence of electrostatic fields. The field is established between the jet and the grounded target and the boiling curves are obtained. The boiling curves showed surprising deviation from typical boiling curves at post-critical heat flux (CHF) regimes. In the absence of electric field, the heat flux always falls suddenly as CHF regime reaches. The CHF phenomenon is a catastrophic phenomenon accompanying with overshooting the surface temperate. However, counter-intuitively, here we demonstrated that applying an appropriate electrostatic field, the curves became flat at post-CHF regimes and critical heat flux regime can be operated at a wide range of superheats. A hypothetical explanation is proposed to rationalize the flattened boiling curve observations at the CHF regime. Flattening of boiling curves in the presence of electric field promotes reliable operation of cooling device at CHF regime.

Capillary column enhanced CHF microgravity flow boiling

Flow boiling heat transfer under microgravity conditions can be extended and enhanced by means of using porous stacks, or capillary columns, arranged on top of a flat heated surface. Under these conditions, body forces are negligible to remove the generated vapor away from the hot surface, which eventually hinders liquid from reaching it. It is possible to increase the critical heat flux (CHF) by having porous stacks symmetrically arranged on this surface; which draws the liquid phase towards it by means of capillary forces. Various flow regimes in the capillary enhanced surface flow boiling can be identified. These include: the regime where the liquid is supplied between the columns, the regime where the liquid flow is controlled by liquid capturing and the viscous drag-capillarity in the columns, and the critical heat flux. For the theoretical model, the expression for the interfacial lift-off model critical heat flux was interpreted based on customizable parameters instead of those imposed by the physics of the flow. This study indicates a potential improvement in CHF by having an inter-column spacing smaller than the critical wavelength for a plain surface. There is also a potential benefit of having the wetting contact to wavelength ratio to be larger than the constant of 0.2 found in experimental studies. The CHF regime can occur by a limitation of the stacks to have access to the liquid phase, as it happens when they are completely submerged in a vapor phase, or by reaching the maximum capillary pressure drop in the stack (as per the Darcy–Ergun momentum equation), or by reaching an entrainment limit of the vapor flow passed the capillary columns. Therefore the critical heat flux can also be extended as long as the capillary columns protrude over the vapor layer and their viscous capillary and entrainment limits are not reached.

Simulation of Boiling Flow Experiments Close to CHF with the Neptune_CFD Code

A three-dimensional two-fluid code Neptune_CFD has been validated against the Arizona State University (ASU) and DEBORA boiling flow experiments. Two-phase flow processes in the subcooled flow boiling regime have been studied on ASU experiments. Within this scope a new wall function has been implemented in the Neptune_CFD code aiming to improve the prediction of flow parameters in the near-wall region. The capability of the code to predict the boiling flow regime close to critical heat flux (CHF) conditions has been verified on selected DEBORA experiments. To predict the onset of CHF regime, a simplified model based on the near-wall values of gas volume fraction was used. The results have shown that the code is able to predict the wall temperature increase and the sharp void fraction peak near the heated wall, which are characteristic phenomena for CHF conditions.

Ramp-rate limitation experiment using induced current method. Part 2: analysis

This paper describes an analysis of ramp-rate limitation experiments performed by a background magnet only without a power supply for the tested cable. Three-strand U-shape cable-in-conduit conductor samples show distinctive ramp-rate limitation phenomena with sensitive transition between no-quench and quench results. Quench experimental results at various ramp rates are explained by a representative induced loop current model. Since the sample length is less than 1 m and the loop resistance through the joint is an order of 1 μΩ , the dominant loop current is induced mostly through the joint. Multiple quench-recovery processes during continuous magnetic field ramp are also explained due to fast recovery of the sample after quench. It is experimentally observed that the strand heat flux condition can be very influential to determine quench-recovery process near the critical heat flux regime of liquid helium.

Experimental investigation of droplet dynamics and heat transfer in spray cooling

Experimental investigation on droplet dynamics and heat transfer in spray cooling was conducted. Water and water solutions with different surfactant additions (sodium dodecyl sulfate) were used to cool a 10 mm in diameter, horizontal copper surface. A multi-nozzle spray system was constructed for studying the effects of mass flux on spray cooling. This multi-nozzle system provides a variable mass flux spray from 0.156 to 1.20 kg/m 2 s with a diameter and velocity variation less than 20%. The incoming and outgoing droplets were characterized in situ with a newly developed laser phase-Doppler anemometry (PDA). It was found that the heat transfer process in spray cooling can be divided into four regimes using the expulsion rate defined as the ratio of local outgoing to incoming mass fluxes. The advantage of the surfactant addition in spray cooling has also been investigated. Besides adding surfactant results in relative small diameters for both impinging and expulsing droplets, the slope of the droplet expulsion rate at the transition to the critical heat flux (CHF) regime also changes sharply with surfactant addition which results in a almost constant heat removal rate near the CHF regime. This character may provide an additional safety mechanism for a heat transfer device to avoid burnout.

Electric Field Effects on Pool Boiling

This paper deals with an experimental study of the influence of a DC uniform electric field on the nucleate boiling heat transfer. EHD effects are quantitatively investigated by performing experiments on various liquids with different properties. In these experiments, n -pentane, R-113, and R-123 are used as working fluids, and the boiling phenomenon takes place on a horizontally oriented copper surface. The experimental results show (1) an enhancement of the nucleate pool boiling heat transfer, (2) an increase of the critical heat flux (CHF), and (3) disappearance of the hysteresis phenomenon. The basic enhancement mechanisms are analyzed and quantified. For nucleate pool boiling and CHF regimes, heat transfer laws based on dimensionless numbers are proposed and determined from the experimental data. The results obtained with the proposed EHD models are in good agreement with experimental results.

NUCLEATE POOL BOILING UNDER DC ELECTRIC FIELD

This article deals with an experimental study of the influence of a DC uniform electric field on the nucleate boiling heat transfer. Electrohydrodynamic (EHD) effects on heat transfer coefficients for dielectric liquids are quantitatively investigated by performing experiments on various liquids with different properties. In these experiments, n-pentane, R-113, and R-123 are used as working fluids and the boiling phenomenon takes place on a horizontal plane copper surface. The experimental results have shown: (1) a threefold increase of nucleate pool boiling heat transfer coefficients, (2) a threefold increase of the critical heat flux (CHF), and (3) the disappearance of the hysteresis phenomenon. For nucleate pool boiling and CHF regimes, heat transfer laws based on dimensionless numbers are proposed. The results obtained by the proposed EHD model are in good agreement with the experimental results.

Ex-vessel boiling experiments: laboratory- and reactor-scale testing of the flooded cavity concept for in-vessel core retention Part I: Observation of quenching of downward-facing surfaces

This paper presents the results of a series of quenching experiments to examine the boiling curve of relative large downward-facing surfaces. The test masses are 61 cm in diameter and the downward-facing surface is either flat or curved. The work is motivated by the need to assess the ex-vessel boiling process for in-vessel core retention. The critical heat flux is found to be approximately 50 W cm −2. The nucleate boiling regime and the critical heat flux regime are found to be characterized by cyclic two-phase flow patterns.

Mechanisms and predictions of burnout in flow boiling over heated surfaces with an impinging jet

A physical model is proposed for the mechanism of boiling burnout in saturated forced convection on heated discs cooled by liquid jets impinging normal to the surface. It is postulated that the relative velocity between the liquid and vapor bubbles in the film governs the burnout process. Two distinct critical heat flux (CHF) regions have been recognized. When the liquid and vapor velocities are of the same order of magnitude, typical of low pressures, burnout results from droplet splash and is surface tension-controlled. At low vapor velocities relative to the liquid, typical of high pressures, burnout appears to result from the separation of the liquid boundary layer, and is controlled by liquid viscosity. Correlation of the model predictions for CHF with the available data for a wide range of liquid-vapor density ratios has provided new insight into the scaling and the splashing rate of droplets. A criterion for transition between the two CHF regimes is also suggested.

Heat Transfer for Fusion Component Applications

A quasi-automated high heat flux flow boiling facility has been developed for the systematic study of critical heat flux (CHF), heat transfer, and two-phase pressure drop. High heat flux research is important in state-of-the-art electronics and fusion component design. For fusion applications, there are practically no low-pressure data for large values of coolant channel length-to-diameter (L/D) ratio (i.e., 100), channel diameters near 1.0 cm, and medium to high heat flux levels (i.e., 100 to 2000 W/cm/sup 2/). A second step is provided to fill this void. Forced flow boiling (with water) quasi-steady experiments have been conducted on uniformly (resistively) heated horizontal copper tubes. The tubes were 1.02 cm in inside diameter and 117.87 cm long. The inlet water temperature was 20/sup 0/C. For a 1.6-MPa exit pressure, measurements of the CHF varied from the annular flow regime (150 W/cm/sup 2/) to the subcooled flow boiling water regime (425 W/cm/sup 2/). The mass velocity was varied form 0.63 to 3.5 Mg/m/sup 2/ . s. At 1.6 MPa, the transition between the annular and subcooled CHF regimes were measured to occur between 1.03 and 1.26 Mg/m/sup 2/ . s. Large axial variations in the Nusselt number were also measured.

On the relation between critical heat flux and outlet flow pattern of forced convection boiling in uniformly heated vertical tubes

For critical heat flux (CHF) of forced convection boiling in uniformly heated vertical tubes, the author recently proposed generalized correlation equations being classified into four characteristic CHF regimes called L, H, N and HP. In the present paper, a study is made on the relation between the abovementioned CHF-regimes and the flow patterns at the tube exit, by employing existing data of flow patterns for uniformly heated tubes.