Initial Wall Temperature Research Articles

Spreading and splashing of liquid film on vertical hot surface by inclined jet impingement

In this paper, the liquid film formed by inclined jet impingement on the vertical hot wall is investigated by a high-speed camera. The effects of the jet velocity (5.5 m/s ≤ u0 ≤ 14.6 m/s, 3804 ≤ Re ≤ 10098) and initial wall temperature (25 ℃≤T0 ≤ 250 ℃) on the spreading and splashing of the liquid film are explored. Although the jet velocity and initial wall temperature have little effect on the spreading velocity of the liquid film, the wetting front position and liquid film area increase. For T0 ≥ 150 ℃, a unique liquid film deflection phenomenon is observed due to the boiling of the liquid film, which results in a substantial amount of splashing. Based on the characteristics of the deflection splashing, three modes are categorized, i.e., annular splashing, banded splashing, and non-splashing. The splashing of the liquid film on the hot surface is owing to the rupture of boiling bubbles, the breakup of surface waves, and the liquid splashing in the boiling zone. The splashing rate of the liquid film is measured, and it is found that an increase in T0 improves the splashing rate significantly, and the splashing rate even exceeds 70 % at T0 = 250 °C.

Water drops on nanofiber-coated substrates: Influence of wall temperature and coating thickness on hydrodynamics and wall heat flux distribution

Heat and mass transport during drop impact or a gentle deposition onto a heated substrate depends on the surface chemistry, morphology, thermal and mechanical properties of the substrate, as well as the substrate temperature. In particular, if a substrate is coated with a porous layer of a wettable material, the drop spreading is accompanied by the imbibition of a liquid into the layer. The wetted area is significantly enhanced in comparison with the area which can be covered by a drop spreading over a bare substrate. As a result, the liquid evaporation rate increases and the evaporation time decreases. Understanding the interaction between the liquid spreading, imbibition and evaporation is important, both for enhancement of heat transfer in cooling applications and for functionalizing of porous media. In our experimental studies, a disk of infrared-transparent calcium fluoride is used as the base substrate. The disk is coated with a submicrometer layer of black chromium nitride and an electrically conductive chromium layer, used for heating of the substrate by electrical current. Nanofiber coating layers are applied on top of the chromium layer using electrospinning. The dynamics of drop spreading is captured by a high-speed camera in a side view, the imbibition is observed with a top view camera, and the temperature distribution at the substrate-coating interface is captured by an infrared camera. Based on the transient temperature field, corresponding heat flux distributions are determined. For the detailed analysis of the interplay between hydrodynamics and heat transfer, the nanofiber coating thickness and substrate temperature have been varied. It has been found that the drop drying time and maximal imbibed area depend non-monotonously on the coating thickness. Changing the initial wall temperature leads to qualitatively different distribution of the heat flux at the substrate surface.

Heat transfer characteristics of oblique jet impingement liquid film cooling in the forced convection mode

Liquid film cooling by oblique jet impingement is widely used in small liquid rocket engine thermal protection. In this work, heat transfer characteristics of oblique jet impingement liquid film cooling in forced convective mode were experimentally investigated to examine the effect of the impingement velocity and the initial wall temperature. The temperature distribution on the rear surface (opposite to the jet) was recorded by a high-frequency infrared camera, and then the heat flux on the front surface was estimated by solving the inverse heat conduction problem. It was found that the lagging effect by thermal diffusion is nonnegligible and can be well described by the penetration time. In addition, thermal diffusion makes the temperature distribution on the rear surface more uniform than the liquid flow path. In contrast, the heat flux distribution has a clear boundary same as the liquid flow path. The length of the wetting front lwf can be fitted as lwf=atn. The constant a and the wetted area A are affected merely by the impingement velocity. The point with the maximum heat flux is located at the impingement point in the forced convective mode rather than the wetting front in the film boiling mode. Inside the liquid film, there is no clear boundary as the film thickness distribution between the thin layer zone and the raised zone, implying the local film velocity determines the heat transfer rather than the film thickness. The cooling power increases with higher impingement velocity and initial temperature difference. The standardization fitting equation shows the effect of the initial temperature difference on the cooling power is more significant than the impingement velocity.

Analysis of transient characteristics of droplets in the DFFB regime during single-rod channel reflood experiment

This study aims to explore the dynamic distribution and hydrodynamic characteristics of droplets during the highly transient process of reflood. However, the quasi-steady state analysis is generally adopted in existing studies reported in the literature, ignoring the dependence of droplet formation and breaking mechanism in the Dispersed flow film boiling (DFFB) mechanism on the time-related historical behaviors such as the propagation of the quench front. A series of reflood cooling visualization experiments of zirconium alloy high-temperature rod-shaped heating surface were carried out to achieve this research goal. The wall surface and fluid temperature history curves were recorded, and the parameters, such as the size and velocity of DFFB regime droplets, were quantitatively extracted by visual image processing technology. Based on the formation and breakup mechanism of droplets in the dispersed flow, the reasons for droplet size distribution and velocity change with input heat flux, initial wall temperature, inlet subcooling, and inlet flow rate are well analyzed and explained. At the same time, the experimental results also show a strong correlation between droplet behavior and the reflood process in time. This study can provide more detailed physical support for the subsequent development of droplet behavior models more suitable for the strong transient reflood process and the evaluation of existing models.

Transient heat transfer during consecutive impact of two droplets on a heated substrate

Spray cooling is an attractive solution to cool high heat flux dissipating surfaces. However, the modelling of its transport process is very challenging due to simultaneous interactions of multiple droplets with the heated wall. In this context, we have performed an experimental investigation of two consecutive droplets impacting on a heated substrate. First droplet (Droplet #1) impacts the hot surface at a certain We number, followed by oncoming droplet (Droplet #2) impacting the same substrate at the same We number, after a designated time (∼ thermal diffusion time scale). Stainless steel foil heater is utilized to provide constant volumetric heat generation, along with a desired initial wall temperature boundary condition. The temperature field of heated SS substrate is altered due to the impact of Droplet #1. Afterwards, Droplet #2 merges with Droplet #1, spreads, recedes, and the combined droplet mass attains a quasi-equilibrium shape. The variation of peak heat flux during impact of Droplet #2 is distinctly lower compared to the initial impact of Droplet #1. Further spreading during impact of Droplet #2 beyond the region of Droplet #1 leads to interaction of liquid with region of higher temperature substrate and results in greater net heat transfer at the peripheral region of the spreading combined droplet mass. The key differences during Droplet #2 impact is delineated compared to Droplet #1 impact on a dry heated substrate. Additionally, the role of We number of Droplet #2 and the initial substrate temperature on the ensuing heat transfer to the droplet is also reported. This study helps in understanding the nuances of such consecutive droplet interaction and its influence on applications such as spray cooling so that better models may eventually be developed, incorporating the experimental details and outcomes.

Experimental study of minimum film boiling temperature and quench front propagation velocity characteristics during the reflooding process

The plate-type fuel assembly stands as a prevalent choice in research reactors and naval power reactors, owing to its expansive heat transfer area and high heat transfer efficiency. Examining the flow heat transfer during the reflooding process and conducting safety analyses pertaining to this configuration emerge as a matter of great significance. Within the confines of this study, the ramifications brought by distinct reflooding velocity, coolant subcooling, heat flux, and initial wall temperature on the minimum film boiling temperature (MFBT) and quench front propagation velocity (QFPV) is delved. By employing a meticulous analysis of the standard regression coefficients, we discern the sensitivity of MFBT and QFPV under each individual condition. The results indicate that reflooding velocity and subcooling meet a significantly positive effect on MFBT and QFPV. The reflooding velocity has the most significant impact on the two key parameters, with an SRC of 0.603 for MFBT and 0.577 for QFPV, respectively. However, the initial wall temperature has a negative effect, while the heat flux does not play a critical role, as indicated by an SRC of only 0.135 for MFBT. The findings presented in this paper offer valuable data support for the safety design of nuclear reactors, and the conclusions drawn from the sensitivity analysis provide refined guidance for future research direction.

Thermal characteristics of melting of a phase change material enhanced by a stainless-steel 304 periodic structure

In this work, melting of a high-temperature inorganic phase change material (PCM) eutectic (with a melting point of 569 °C) within a vertical cylindrical tank has been experimentally investigated. To promote the heat transfer rate, a periodic structure that is constructed by a commercial SS-304 mesh screen has been considered and immersed into the PCM tank. Thermal characteristics of the PCM-periodic structure tank under different initial temperatures (450, 490 and 546 °C) and wall temperatures (620, 640, 660, 680 and 700 °C), are then investigated and reported. The presented experimental data can facilitate practical engineers to find the best operating condition of similar PCM tanks; meanwhile, it can also be employed for the investigation of thermal response of transient heat conduction before melting starts.

Chrono-photographic visualization and characterization of the flow regimes in rewetting by bottom flooding

Many engineering applications encounter the highly complicated rewetting phenomenon. The complexity of the process arises as the phenomenon is highly transient and possesses an implicit interplay of several transport processes. In particular, the hydrodynamics of in-conduit rewetting is poorly understood due to lack of visualization. The present study reports an in-depth quantitative representation of the hydrodynamics of different two phase flow regimes associated with rewetting by bottom flooding, perceived from an intelligently fabricated experimental test facility. The technique of chronophotography and image processing based on pixel level information of a fixed region of interest has been utilized to quantitatively characterize different flow regimes like – nucleate boiling, transition boiling, inverted annular film boiling and dispersed flow film boiling. The effects of inlet mass flux and initial wall temperature have been explained based on operating regime maps. The rich hydrodynamics captured in this report provides an in-depth knowledge of the associated heat transfer mechanisms.

A general prediction model of minimum film boiling temperature during quenching propagation in narrow rectangular channel

During quenching, the Leidenfrost effect deteriorates the heat transfer at high temperatures before the arrival of the minimum film boiling point. Accurate prediction of the minimum film boiling temperature (Tmin) is significant in assessing cooling efficiency and providing a reference for optimization design. The experimental investigation on the Tmin is carried out in multi-narrow rectangular channels containing internal and external coolant channels. The effects of initial surface temperature and wall heat flux on Tmin are discussed in detail. The experimental results indicate that Tmin increases with the increase of initial wall temperature and wall heat flux, by an average of 6.5% and 7.7%, respectively. Under high wall temperature and heat flux conditions, the ratio of Tmin to the initial wall temperature at the inlet is much larger and then remains nearly constant with the propagation of the quench. Furthermore, an improved dimensionless prediction model of Tmin is proposed considering the effects of high initial wall temperature and wall heat flux. The comparison results illustrate good agreement with relative errors within ± 15%. Meanwhile, the current model predicts most of the published experimental data with an uncertainty of ± 15%.

Analytical analysis of non-boiling heat transfer during droplet impact on heated wall and the effect of thermophysical properties

Droplet impact process exhibits excellent heat and mass transfer characteristics. Analytical solution with simple explicit form and certain accuracy can clearly reflect the correlation of physical parameters, however the analytical study on droplet impact heat transfer is quite insufficient and limited to flow field analysis. In this paper, the analytical solutions of heat flux and temperature variation of droplet impact on heated wall in non-boiling regime were obtained based on boundary layer asymptotic matching method. Parameter analysis of thermophysical properties was conducted to explore the decisive properties of droplet and wall materials. The results showed that the analytical solutions are appropriate to predict non-boiling heat transfer at low impact velocity and low initial wall temperature. Parameter analysis showed that thermal effusivity is the decisive property rather than the thermal diffusivity. Wall thermal effusivity could independently determine heat flux, but should determine temperature variation together with wall thermal conductivity. Droplet thermal effusivity and Prandtl number were the decisive properties, however the effect of Prandtl number in reality can be ignored. Wall materials and droplet liquids with higher thermal effusivity showed higher heat transfer capacity. The importance of thermal effusivity was demonstrated and high thermal effusivity was preferred in future thermal management.

EFFECTS OF WALL TEMPERATURE ON THERMOPHYSICAL CHARACTERISTICS OF SPRAY-WALL IMPINGEMENT

For small- and medium-bore diesel engines, the phenomenon of spray-wall impingement is unavoidable, which affects the evaporation of the fuel, thermal efficiency, and emissions. Wall temperature is an important parameter in this process because it has an important influence on fuel evaporation. In this paper, the thermophysical characteristics of spray-wall impingement at different wall temperatures were studied. The results showed that with increase of the initial wall temperature, the impingement spray radius increased, but its rate of increase decreased, while the impingement spray height first increased and then decreased. During this process, the surface heat flux first increased and then decreased with the increase of initial wall temperature, which was due to the change of heat transfer regime on the wall. The transient heat transfer process at different initial wall temperatures was characterized by the Biot and Fourier numbers and the dimensionless surface heat fluxes were highly similar. A dimensionless correlation was developed to quantify the transient heat transfer process, which showed that the ratio of the internal thermal resistance of heat conduction in the wall to the wall surface convective heat transfer resistance changes almost linearly during the process of spray-wall impingement.

Experimental study of the factors on solid adsorption vacuum flash evaporation for ice production

The paper based on the solid adsorption vacuum flash ice slurry production device, the process of distilled water flashing was recorded with a high-speed CCD camera, and the vacuum flashing state of distilled water at different pressures and different initial temperatures was analyzed by experimental phenomena. The effect of system pressure, different wall temperatures and solution volumes on the subcooling state of the ice slurry formation process under solid adsorption vacuum flash evaporation was also experimentally investigated. The results show that the lower the pressure the more intense the flashing phenomenon in the vacuum flashing state, and the higher the initial temperature the more likely the solution is to produce bubbles. Within 20 seconds after the start of the experiment, distilled water will appear subcooling phenomenon in solid adsorption vacuum flash evaporation ice making system, due to the vacuum flash action water is constantly disturbed subcooling state duration is very short. The lower the pressure, the lower the subcooling degree, the subcooling degree is about 2 °C - 4 °C. The wall temperature has a greater impact on the ice crystal phase change accumulation stage, the higher the initial wall temperature the greater the fluctuations in the subsequent stages of flashing and the lower the ice content. When the volume of the solution is small, the liquid film is prone to flash formation of solid ice.

Quenching experiments of vertical Inconel and Zircaloy tubes in internal water flow

In this study, quenching experiments of vertical Inconel and Zircaloy tubes in internal water flow were conducted. The effects of the initial tube wall temperature, liquid subcooling, and liquid injection velocity on the quench temperature, the reaching time to quench temperature, and the quench front velocity were examined. For the experimental conditions, the initial tube wall temperatures were 700 and 1000 °C, the liquid subcoolings were 10, 50, and 75 °C, and the liquid injection velocities were 0.05 and 0.1 m/s. An increase in the initial tube wall temperature increased the quench temperature and prolonged the reaching time to quench temperature with the decrease of the quench front velocity. An increase in liquid subcooling shortened the reaching time to quench temperature and increased the quench temperature and quench front velocity. In addition, an increase in the liquid injection velocity reduced the reaching time to quench temperature, and increased the quench temperature and quench front velocity. For all cases, the Inconel tube exhibited a longer reaching time to quench temperature with lower quench temperature and quench front velocity than those of the Zircaloy tube. Based on the present experimental data for the quench temperatures of Inconel and Zircaloy tubes, the previous correlations were compared and assessed. The correlation of Kim and Lee (1979) predicted the quench temperature data with reasonable accuracy.

The cooling performance of the micro-solid nitrogen spray technique on the cryopreservation vitrification process: A qualitative study

The qualitative mechanism of heat transfer during the cooling process by spraying Micro-Solid Nitrogen (MSN2) together with the capability to vitrify a large-volume of samples is experimentally investigated in the article. The experiments were carried out on three samples: one was a 30 30 × 0.1 mm copper plate attached to a 6.75 ml container filled with distilled water, the second was a 25 × 25 × 3 mm ceramic heater with a 15 × 15 × 0.4 mm heat flux sensor installed on top, and the third with a recess of 10 × 10 × 1 mm to test the vitrification capabilities of large-volume biological samples. Thermal characteristic experiments were performed for three initial wall temperatures of 278, 313, and 373 K, respectively. The cooling rate achieved by the MSN2 spray ranges from between 3.97 K/s up to 47 K/s and depends on the specific wall temperature range. The results of the experimental data show an average three times higher value of heat flux by using MSN2 compared to the boiling pool of Liquid Nitrogen (LN2) within the same temperature range. The investigation exposed the second local maximum of heat flux in the wall superheat temperatures between 30 K and 300 K, which is shifted from the Critical Heat Flux (CHF) point towards higher temperatures. The vitrification of the sample was achieved using a cell-free liquid solution with 7 mol/L for both dimethyl sulfoxide (Me2SO) and glycerin agents. The average cooling rate for the vitrified samples during the experiments ranges from between 2 - 4 K/s and is about 3 to 6 times higher than the cooling rate of similar volume samples known from the literature.

Experimental Investigation of AdBlue Film Formation in a Generic SCR Test Bench and Numerical Analysis Using LES

In this work, an experimental investigation of AdBlue film formation in a generic selective catalytic reduction (SCR) exhaust gas test bench is presented. AdBlue is injected into a generic SCR test bench resulting in liquid film formation on the lower wall of the channel. The thickness of this liquid film is measured using a film thickness sensor based on absorption spectroscopy. Simultaneously, the wall temperature at the measurement point is monitored, which allows for examining correlations between the evolution of the film thickness and the temperature of the wetted wall. The velocity of the airflow in the channel and the initial wall temperature are varied in the experiments. Correspondingly, the measurements are performed during different thermodynamic regimes, including liquid film deposition and boiling. Repeated measurements have also shown that the film thicknesses are reproducible with a standard deviation of 3.4 %. LES-based numerical simulations are compared to the experimental results of the film thickness during the early injection stage. Finally, a numerical analysis is performed to analyze the AdBlue droplet impingement and subsequent film-formation dynamics.

Experimental investigation on minimum film boiling temperature during reflooding in multi-rectangular narrow channels

Determination of minimum film boiling temperature (Tmin) is significant in the nuclear reactor safety analysis. Tmin represents the start of quenching which is vital to mitigate the severity of the accident. In this paper, a bottom reflooding experiment is carried out to investigate Tmin and quench velocity in multi-rectangular narrow channels. Effects of working conditions, including inlet subcooling, reflooding velocity, initial wall temperature, and system pressure on Tmin and quench velocity, are analyzed in detail. It is found that Tmin and quench velocity distinctly increases with the increase of subcooling, system pressure, and reflooding velocity. However, the effect of reflooding velocity on Tmin and quench velocity is mild when the reflooding velocity is relatively high. Tmin increases with the increase of initial wall temperature, while quench velocity drops. Tmin at the upper position of the heating wall is lower than that at the lower position because the water subcooling changes when flowing along the heating channel. A prediction model of Tmin during reflooding in rectangular narrow channel is proposed, and very good agreement is achieved comparing with experimental data with ±10% uncertainty.

Efficient Evaluation of Heat Exchangers Behaviour in Nuclear Power Plants

This paper studies the performance of the primary and secondary flow paths of the Heat Exchangers (HEXs) in the Nuclear Power Plants (NPP). Explicit solutions for the wall temperature, primary and secondary wall temperatures of the HEX are presented. Therefore, closed form expressions for wall temperature, primary and secondary wall temperature are proposed. Moreover, performance analysis of HEX is investigated. Curves for the HEX wall temperature are introduced with special emphasis on the initial wall temperature. The various effects of pipe wall heat capacities, perimeter, fluid Heat Transfer (HT) coefficient and wall HT coefficient on the HEX are evaluated. The obtained results confirm that the surface contact between the two fluid passes reduces the temperature to some extent at 5 seconds. Furthermore, it is limited to the pipe length and contact time. As a final point of observation, the distributed temperature through the HEX structure increases with the decrease of perimeter. Since, the wall area decreases with the decrease of perimeter. The proposed work allows more control of the HT process within the HEX. Consequently, the performance of NPPs can be improved.

Experimental Investigation in the Autoignition Behaviour of Aviation Jet Fuel in a Rapid Compression Machine

Jet A-1 fuel’s autoignition behaviour was examined experimentally in a newly developed rapid compression machine (RCM) at the University of Sheffield. The experiment was conducted at a compressed temperature at compressed temperatures of 697 K ⩽ Tc ⩽ 796 K, compressed pressure of 6 and 10 bar, and under lean and stoichiometric conditions (φ = 0.75 and 1.0). The initial chamber wall temperature was varied from 115°C - 135°C. The effect of the compressed temperature, compressed pressure and equivalence ratio was seen to have influence on the ignition delay time (IDTs) of Jet A-1. In the experimental condition studied, Jet A-1 exhibited an Arrhenius behaviour that is increasing the compressed temperature, compressed pressure and equivalence ratio decreases monotonically with IDTs. Evidence of two-stage ignition delay and absence of Negative Temperature Coefficient (NTC) behaviour was observed within the condition studied.

Quenching Experiments with CrAl-coated Zircaloy Cladding in Reflooding Water Flows

A quenching experiment is performed to investigate the heat transfer characteristics and cooling performance of CrAl-coated Zircaloy (Zr) cladding in a water flow. The CrAl-coated Zr cladding is one of the accident tolerant fuels for light water reactors. The uncoated Zr cladding is also used in this quenching experiment for comparison. This experiment simulates reflood quenching of fuel rod during loss of coolant accident (LOCA) in nuclear power plant. The test conditions were determined to represent the peak cladding temperature, the coolant subcooling and the reflood velocity in the event of LOCA. The flow visualization showed the film boiling during early stage of reflood quenching and the transition to nucleate boiling. The film layer decreases as the coolant subcooling increases and becomes wavy as the reflood velocity increases. The CrAl-coated Zr cladding showed more wavy and thinner film than the uncoated Zr cladding. The rewetting temperature increases as the initial wall temperature and/or the coolant subcooling increases. The quench front velocity increases significantly as the coolant subcooling increases. The reflood velocity has a negligible effect on rewetting temperature and quench front velocity.

Transient, Two-Dimensional Heat Conduction Model for Rewetting a Hot Vertical Plate Surface by a Falling Liquid Film.(Dept.M)

A transient, two-dimensional heat conduction model for rewetting a hot plate surface by a falling liquid film is developed. It is assumed that the plate thermal properties are Independent on temperature, and the heat transfer coefficient is a constant value in the wet region and zero in the dry region. The mathematical model equations are transformed to a dimensionless form solved numerically . The results indicate that the heat conduction problem in the rewetting process is substantially two-dimensional one, not only for thick plates, I.e., a large Bolt number, but also for a low initial wall temperature. An explicit simple formula for predicting the rewetting velocity is obtained by correlating the numerical results, Comparisons with other studies are made.