Occurrence Of Dryout Research Articles

Dryout prediction with CFD model of annular two-phase flow

Two-phase flow and heat transfer are of interest to industrial applications due to its high efficiency. In a diabatic annular two-phase flow, the liquid film is depleted by both entrainment of liquid droplets and by evaporation. When the liquid film experiences almost complete depletion and cannot cover the wall, the heat transfer between the fluid and the channel wall significantly deteriorates, leading to the onset of boiling transition called dryout. While the dryout is milder than the departure from nucleate boiling (DNB) occurring in low quality two-phase flows, it could still challenge and damage the channel wall. As a result, the dryout occurrence needs to accurately predicted and avoided in practice, such as in boiling water reactors (BWRs). Research interests haven been recently focused on dryout prediction with annular flow modeling, with three fields of gas, droplets and liquid film accounted for. In the current study, one unified computational fluid dynamics (CFD) model for annular flow was developed for dryout applications. The model is employing a separate solver of two-dimensional conservation equations to predict propagation of a thin boiling liquid film on solid walls. The film model is coupled to a solver of three-dimensional conservation equations describing the gas core, which is assumed to contain a saturated mixture of vapor and liquid droplets. All the major interaction phenomena between the liquid film and the gas core flow have been accounted for, including the liquid film evaporation as well as the droplet deposition and entrainment. The resultant unified framework for annular flow has been applied to the steam-water flow with conditions typical for a BWR. The simulation results for the liquid film flow and dryout occurrence show favorable agreements with the available experimental data.

Flow boiling of R452A: Heat transfer data, dry-out characteristics and a correlation

This paper presents an experimental investigation on two-phase heat transfer and dry-out occurrence for refrigerant R452A in a single horizontal circular stainless-steel tube having an internal diameter of 6.0 mm. The effects of mass flux (from 150 to 600 kg/m2s), saturation (bubble) temperature (from 23 to 55 °C) and heat flux (from 10 to 65 kW/m2) are investigated and discussed. Heat transfer coefficient and dry-out vapor quality data are then compared to R404A results in the same operating conditions, observing that the nucleate boiling contribution of the new blend is penalized by its very high temperature glide during evaporation. The assessment of some dry-out and flow boiling heat transfer coefficient prediction methods is finally carried-out and a correction factor on the nucleate boiling term is proposed to take into account the negative effect of the temperature glide difference on the mass diffusion in the liquid. By implementing this modification on two chosen asymptotic models, the statistical error analysis is considerably improved.

Numerical calculation of the thermal-hydraulic characteristics in a once-through steam generator based on a coupled boundary

The operational processes of the primary side, secondary side, and heat transfer tubes of a once-through steam generator were numerically modeled through conservation equations for the flow and heat transfer processes, where the dryout criterion was modified to extend the application range of the model to accurately portray coupled heat transfer conditions. The results show that the proposed model and coupled method was used to accurately calculate the actual operation. The secondary wall temperature gradient at dryout position, based on a coupled boundary, is approximately several tens of degrees Kelvin per meter (K/m), while the value using a constant heat flux method reaches several hundreds of degrees Kelvin per meter (K/m). The primary and secondary pressures decrease approximately linearly and non-linearly along their own flow directions, respectively. The occurrence of dryout leads to a deviation from thermodynamic equilibrium in the post-dryout region, where the actual steam quality rises slowly with a smaller gradient compared to that in the pre-dryout region. Therefore, the acceleration of steam-liquid has less effect on total pressure drop and on the decrease in secondary pressure gradient.

Flow boiling heat transfer and pressure drop of pure ethanol (99.8%) in a horizontal stainless steel tube at low reduced pressures

Anhydrous ethanol is a potential working fluid for some applications, such as heat pipes and ORC systems. The current work presents two-phase experiments on flow boiling heat transfer and frictional pressure drop of pure ethanol in a horizontal stainless steel tube (type SS316) having an internal diameter of 6.0 mm. The influence of operating conditions in terms of mass flux, saturation temperature and imposed heat flux have been investigated. Particularly, the mass flux was fixed from 85 to 127 kg/m2 s, the saturation temperature was set from 64.5 up to 85.8 °C (corresponding to reduced pressures of 0.009 and 0.021) and the heat flux was varied from 10.0 to 40.3 kW/m2. According to the Taitel and Dukler flow pattern transition method (Taitel and Dukler, 1976), all the experimental points fall within the annular flow regime. Consistently, the average heat transfer coefficients show a typical convective behavior, being affected only by the mass flux and showing a significant increase with vapor quality and up to the dry-out occurrence. The frictional pressure gradients were instead seen to be increased with higher mass fluxes and lower saturation temperatures.Among the correlations chosen for comparison, the pure convective heat transfer model of Cioncolini and Thome (2011) developed for annular flow and the pressure drop correlation of Friedel (1979) return the best predicting accuracy (MAE equal to 25.3% and 22.3%, respectively).

Average liquid film thickness of annular air-water two-phase flow in 8 × 8 rod bundle

In order to develop detailed model of the annular two-phase flow the average liquid film thickness is an important parameter. It can significantly affect the occurrence of dryout and post-dryout phenomenon on heater surfaces. Most research on film thickness has been focused on pipe flows. Data in rod bundle geometry are very limited. However data in rod bundle geometry is much more valuable for reactor safety modeling and calculations. Because of this, an experiment to measure liquid film thickness has been performed for the air-water annular flow in a 8 × 8 BWR rod bundle. Film thickness data were obtained both on the rods and on the channel wall. Data were recorded at four axial locations within the rod bundle. This included locations just before and just after a spacer grid. This allowed the spacer grids effect on the film thickness to be evaluated. The minimum film thickness was on the center rod. Also, the presence of the spacer grid resulted in reduced film thickness.

Heat Transfer Performance Enhancement of Gravity Heat Pipes by Growing AAO Nanotubes on Inner Wall Surface

In this paper, the heat transfer performance of gravity heat pipes with anodic aluminum oxide (AAO) wall surface is studied. The main purpose is to study the effects of the length and diameter of AAO nanotubes on the temperature distribution, overall thermal resistance, and dry-out occurrence of gravity heat pipes charged with acetone under different input heat powers. AAO nanotubes were first grown by anodizing the inner wall surface of the evaporator section of aluminum alloy gravity heat pipes. The influences of AAO nanotube length and diameter on the temperature distribution, overall thermal resistance, and dry-out occurrence were then investigated by a thermal performance test system. Experimental results show that increasing the AAO nanotube length could result in reduced temperature variation between the evaporator section and the condenser section, leading to reduced thermal resistance, and delayed dry-out occurrence at higher heat inputs. In addition, increasing the AAO nanotube diameter could also cause decreases in temperature variation and overall thermal resistance, but it could not have a significant effect on the occurrence of dry-out phenomenon. Based on these results, it can be concluded that, if the anodic oxidation treatment is applied to the inner wall surface of the evaporator section of a gravity heat pipe, its heat transfer performance could be significantly improved. The maximum temperature difference and overall thermal resistance of the processed heat pipe are 46.12% and 58.68% lower than those of the unprocessed heat pipe, respectively; moreover, compared to the unprocessed heat pipe, the maximum applicable input heat power to avoid dry-out occurrence can be increased up to about 40%. Such a study could be used for cooling purposes in a wide range of applications such as passive cooling of electronic devices, highly efficient heat recovery, and cleanroom air conditioning.

Experimental study on the dryout point and post-dryout heat transfer in square channel

Blanket is one of the most important safety components for heat transfer in fusion reactor. Under accident conditions, two-phase fluid boiling flow in square channel of blanket is extremely efficient for cooling of fusion reactor core. The second kind of heat transfer deterioration usually occurs when the quality is high such as annular flow in the boiling process. The liquid film is depleted for the entrainment of liquid droplets and the evaporation, and no longer covers the wall. The heat transfer between the fluid and the channel wall deteriorates, leading to the boiling crisis called dryout (DO) and a rapid rise in wall temperature when the wall heat flux reaches critical heat flux (CHF), which could damage the channel wall. Therefore, accurate prediction of dryout occurrence and the post-dryout heat transfer are crucial to the optimized design and safety operation of fusion reactor. For square channel in blanket, the experimental study of dryout has been conducted. Experimental results show that the dryout heat flux increases with the increase of mass velocity and with the decrease of inlet vapor quality. Furthermore, with the increase of heat flux, the location of dryout point move to inlet of test section and the wall temperature in dryout point increases gradually. Based on the obtained experimental data, the correlation for prediction of critical vapor quality (XDO) in the square channels is proposed, which is compared with the predictions obtained by other correlations.

Experimental study of hybrid loop heat pipe using pump assistance for high heat flux system

Loop heat pipe is a promising thermal control system, the same as a heat pipe, to realize the efficient release of heat from electric appliances. It is suitable for applications in electronics that have higher power density. Meanwhile, the heat pipe loop (LHP) can also be considered for adoption in the manufacture of solar hot water systems, due to its unique features such as effective thermal conductance and flexible design embodiments. However, the start-up problem of the LHP and transport distance and higher heat release capacity is still existed influencing the operating stability of the device. Base on the problem, this study focuses to carry out work on the developing a novel LHP system in order to provide a robust solution for significant enhance the ability energy transfer. This novel LHP is a conventional LHP that was modified by adding a diaphragm pump to accelerate the fluid transportation (called as hybrid loop heat pipe, HLHP). The pump is installed on the liquid line complete with a reservoir. It will work passively using the wick capillary pressure when there is no sign the occurrence of dry-out. In another hand, the pump was only activated when the evaporator temperature tends increased extremely because of the failure of start-up. The experimental result showed that installed diaphragm pump in LHP modified system was able to prevent the occurrence of dry-out and significantly reduced the evaporator temperature. This study will contribute to energy savings and the utilization of renewable energy.

Effects of heat flux, mass flux and two-phase inlet quality on flow boiling in a vertical superhydrophilic microchannel

Saturated flow boiling experiments were conducted to investigate the influence of surface wettability on the hydraulic and thermal transport performance in a large width-to-height aspect ratio, one-sided heated rectangular microchannel with deionized water as the working fluid. The contact angles of the bare silicon wafer surface and superhydrophilic surface after deposited by a thin film of 100-nm-thickness silicon dioxide were 65° ± 3° and less than 5° respectively, both of which were utilized as heated surfaces of the microchannel. Parametric experimental studies were carried out with the inlet vapor quality varied from 0.03 to 0.1 and the wall heat fluxes spanned from 4 W/cm2 to 20 W/cm2, at various mass fluxes ranging from 120 to 360 kg/m2 s. High speed flow visualizations were conducted coupled with instrumental measurements to illustrate the effects of heat flux, mass flux and two phase inlet quality on the local heat transfer coefficient, averaged heat transfer performance, two phase flow structure and pressure drop characteristics for surfaces with distinct surface wettability characteristics in the microchannel. Experimental results showed that the local heat transfer coefficient decreased first until approaching a minimum value and then increased towards the exit along the flow direction. Severe heat transfer deterioration was obtained for the bared silicon wafer surface with increased inlet vapor quality and heat flux, resulting from the local dryout phenomenon as can be verified by the flow visualization. While the heat transfer performance of the superhydrophilic surface was relatively constant due to continuous and uniform distribution of the thin liquid film on the heated surface during annular flow dominance and subsequent delay to partial dryout occurrence, which outperformed the untreated hydrophilic surface without additional pressure drop penalty.

Numerical Investigation of Water/〖Al〗_2 O_3 Nanofluid Dryout Phenomenon in a Vertical Channel

Critical heat flux has been recognized as the upper limit for the safe operation of many cooling systems which may lead to the occurrence of dryout causing a large temperature gradient in the heated wall. One way to increase the amount of the critical heat flux is to put in nanoparticles such as Al2O3 to the base fluid. The current research investigates the nanoparticles effect on dryout phenomenon using computational fluid dynamics. Boiling phenomena are simulated using the mechanistic model organized in Rensselaer Polytechnic Institute which is extended to analyze the critical heat flux by partitioning wall heat flux to liquid and vapor phases. It was shown that the dryout phenomenon can be delayed by increasing the nanoparticles concentration, and in certain concentration of nanoparticles (5 percent), dryout would not take place.

Two-phase flow characteristics of a high performance loop heat pipe with flat evaporator under gravity

Aiming to fully investigate the two-phase flow characteristics inside a loop heat pipe (LHP), a visualization study for a compact copper-water loop heat pipe with flat evaporator was carried out under gravity favorable mode in this work. Under forced water cooling condition with the cooling water temperature of 25 °C, the results indicated that the compact LHP could efficiently dissipate a maximum heat load of 550 W with the evaporator temperature of 91.2 °C. A minimum LHP thermal resistance was 0.068 °C/W, which was obtained at 500 W. In addition, the evaporator temperatures at boiling incipience under different heat loads were evaluated, which showed slight dependence on the input heat load. Finally, the flow patterns during the startup stage and the steady stage associating with different heating loads were recorded and analyzed to investigate the internal operation mechanism of the LHP. As a result, various two-phase flow patterns were found to be related to the heat load, and the occurrence of partial dry-out was observed in the evaporator at the maximum heat load of 550 W, corresponding to a heat flux of 88 W/cm2.

Experimental investigation of a stainless steel two-phase closed thermosyphon

An experimental investigation has been carried out to study the heat transfer characteristics of a stainless steel two-phase closed thermosyphon. Three compatible working fluids were tested: water, ethanol and acetone. Since the filling ratio is expected to have a predominant effect on the steady-state thermal performance of a thermosyphon, three cases were considered: the underfilled, overfilled and optimally-filled sets. The effects of the input heat flux at the evaporator and the cooling fluid temperature at the condenser were also investigated. The steady state wall temperature was then monitored at various locations along the thermosyphon. The obtained results showed that the dry-out occurrence at the evaporator bottom resulted in an obvious temperature rise, especially for the underfilled case. The experimental heat transfer coefficient at the evaporator was estimated and compared with pool boiling correlation data. A good agreement between the experimental results and the prediction values was obtained for ethanol. A reasonable agreement was found for water and acetone.

Experimental study on a hybrid loop heat pipe

A conventional loop heat pipe two-phase heat transfer device of passive system often can no longer meet the challenging cooling needs due to the inherent limitations of the capillary pumping which can lead to dry out. This study aims to create a loop heat pipe uses capillary wick copper sintered with centrifugal casting method. The stressing effort to overcome the dry-out by adding a diaphragm pump to accelerate the fluid transportation from the condenser to the evaporator ( hybrid loop heat pipe, HLHP ), where the pump is equipped with a reservoir and both installed on the liquid line. In testing the performance of HLHP also varying the filling ratio, FR : 50%, 60%, and 80%. The pump will be activated when the dry-out took place, by the piezo electric diaphragm pump with temperature controller installed in the evaporator that was set to activate the pump to work. From the results of the experimental, the pump successfully prevented the occurrence of dry out, and reduced the temperature of the evaporator from 130°C to 80°C, owing the pump distributed the working fluid from the condenser to the evaporator efficiently. The result indicated the best performance of HLHP was filling ratio, FR of 60%.

Effect of the Dryout in Tube Bundles on the Heat Transfer Performance of Falling Film Evaporators

In this paper, the dryout occurrence was discussed on the tube bundles in falling film evaporators for refrigeration. A numerical simulation was carried out on the bundles of horizontal tubes based on the distributed parameter method. In the simulation, the liquid flow rate and maldistribution on the tube bundles were analyzed about their influences on the dry patch formation and heat transfer performance of evaporators. These results are helpful to guide the design of falling film evaporators.

Numerical prediction of dryout in a 19 rod bundle under the effect of eccentricity and blockage

An accurate estimation of dryout power and its location (zcr) is central to the safety of nuclear reactors. In the present study, a sub-channel analysis code is developed by extending the standard single phase DIANA algorithm to two phase flow conditions. The mass, momentum and energy conservation equations are solved, using a mixture model, which is validated against available experimental data. Numerical simulations are performed to determine the dryout location for a circular 19 rod bundle, in conjunction with a film thickness model. In critical sub-channels, a sudden jump in wall temperature was noticed at the dryout location. The effect of eccentricity(e) on the dryout location was investigated in the range of 0.0⩽e⩽0.7, under different operating conditions. It was observed that, eccentricity causes flow maldistribution in different sub-channels, and in turn affects the dryout location. For low inlet mass fluxes, sub-channels which never experienced dryout (for e=0.0) were found to experience dryout (for e>0.0) The effect of blockage (b) was also systematically studied for 0.0⩽b⩽0.3. In flow regimes with higher vapor quality, the blockage leads to a disturbance in the continuous liquid film, resulting in an early occurrence of dryout. Two types of axial power distribution (APD) viz uniform and sinusoidal heat flux imposition were numerically simulated. The latter was found to delay the occurrence of dryout, compared to the former.

New models of droplet deposition and entrainment for prediction of liquid film flow in vertical annuli

Droplet deposition and entrainment significantly affect dryout occurrences in annular two-phase flow. Currently, most of the correlations for droplet deposition and entrainment are derived from flows in tubes. However, the main challenge in thermal-hydraulic analyses of BWRs is the prediction of dryout occurrence in the rod bundle geometry of fuel assemblies. In the work reported here, the applicability of several tube-based droplet deposition and entrainment correlations to the annular geometry (effectively a “one-rod” bundle) is checked. It is found that the prediction values from all those correlations deviate from the experimental measurements. Therefore, a set of simple and easy-to-implement models for deposition and entrainment rates in annuli is developed based on the tube-based Hewitt-Govan correlations (Hewitt and Govan, 1990), with considering the liquid film flow characteristics in annuli and the geometry effect of the annulus. The present models are verified by comparison with experimental data for developing liquid films in adiabatic annuli. By using these models to predict dryout in heated annuli, we find that these new relationships have good capability in predicting dryout occurrence in annuli with uniform as well as non-uniform axial power distribution.

The effect of self-leveling on debris bed coolability under severe accident conditions

Nordic-type boiling water reactors employ melt fragmentation, quenching, and long term cooling of the debris bed in a deep pool of water under the reactor vessel as a severe accident (SA) mitigation strategy. The height and shape of the bed are among the most important factors that determine if decay heat can be removed from the porous debris bed by natural circulation of water. The debris bed geometry depends on its formation process (melt release, fragmentation, sedimentation and settlement on the containment basemat), but it also changes with time afterwards, due to particle redistribution promoted by coolant flow (self-leveling). The ultimate goal of this work is to develop an approach to the assessment of the probability that debris in such a variable-shape bed can reach re-melting (which means failure of SA mitigation strategy), i.e. the time necessary for the slumping debris bed to reach a coolable configuration is larger than the time necessary for the debris to reach the re-melting temperature. For this purpose, previously developed models for particulate debris spreading by self-leveling and debris bed dryout are combined to assess the time necessary to reach a coolable state and evaluate its uncertainty. Sensitivity analysis was performed to screen out less important input parameters, after which Monte Carlo simulation was carried out in order to collect statistical characteristics of the coolability time. The obtained results suggest that, given the parameters ranges typical of Nordic BWRs, only a small fraction of debris beds configurations exhibits the occurrence of dryout. Of the initially non-coolable configurations, a significant portion becomes coolable due to debris bed self-leveling.

Dryout analysis of overloaded microscale capillary-driven two-phase heat transfer devices

Dryout occurrence at high heat input is one of the detrimental factors that limit the thermal efficiency of a phase-change heat transfer device. In this work, we demonstrate that by employing visualization method, the dryout occurrence of an elongated liquid droplet in a transparent evacuated microscale two-phase flow device can be scrutinized. The circulation of liquid from the condenser to the evaporator is driven by the capillary action which is the primary limitation that governs the maximum heat transport capability of the device. When the evaporation rate exceeds the circulation rate of condensate, dryout will take place in the evaporator end. The propagation of dryout lengths can be accurately determined directly from visualization and a more accurate evaluation of the dryout length compared to the conventional method by measuring the axial temperatures has been developed. By quantifying the performance indicators of the cooling device over a wide range of operating conditions, including the underloaded and overloaded operations, the observation of dryout occurrence in this study correlates highly with the anticipated heat transfer characteristics of a phase-change heat transfer device. This study provides essential insights, particularly on the overloaded conditions, to the design of a microscale two-phase heat transfer device.

Modeling of annular two-phase flow using a unified CFD approach

A mechanistic model of annular flow with evaporating liquid film has been developed using computational fluid dynamics (CFD). The model is employing a separate solver with two-dimensional conservation equations to predict propagation of a thin boiling liquid film on solid walls. The liquid film model is coupled to a solver of three-dimensional conservation equations describing the gas core, which is assumed to contain a saturated mixture of vapor and liquid droplets. Both the Eulerian–Eulerian and the Eulerian–Lagrangian approach are used to describe the droplet and vapor motion in the gas core. All the major interaction phenomena between the liquid film and the gas core flow have been accounted for, including the liquid film evaporation as well as the droplet deposition and entrainment.The resultant unified framework for annular flow has been applied to the steam-water flow with conditions typical for a Boiling Water Reactor (BWR). The simulation results for the liquid film flow rate show good agreement with the experimental data, with the potential to predict the dryout occurrence based on criteria of critical film thickness or critical film flow rate.

Prediction of dryout and post-dryout heat transfer using a two-phase CFD model

This study focuses on development of an integrated CFD model for diabatic high quality two-phase flow including tarns-dryout regions from annular-mist regime to mist regime. One unified three-field CFD model accounting for droplets, gas, and liquid film was developed to simulate both pre and post dryout regions, with local models to determine the dryout occurrence. The thin liquid film model was coupled to the gas core flow model, which is described using the Eulerian–Eulerian approach. For the post-dryout region, the various heat and mass transfer mechanisms between the wall, the gas phase, and the droplets were identified, including the wall-gas convective heat transfer, the droplet evaporation, the droplet-wall direct contact heat transfer and the thermal radiation, to calculate the temperature of the wall and the fluid. Of the most interests, dryout location and wall temperature measurements from a post-dryout heat transfer experiment have been used for the validation. Simulation results show that the dramatic temperature excursion could be well captured using current models. Nevertheless, more work will be continued to improve the accuracy of the results.