Rise Of Heat Flux Research Articles

Effect of twisted tapes and surface extensions in enhancing the condensation heat transfer of high-temperature saturated steam in a circumferential rectangular channel

This article numerically investigates the impact of twisted tapes and extended surfaces (fins) on enhancing high-temperature steam flow in a circumferential rectangular channel, akin to the external jacket cavity of a tyre cure press. The passive enhancement techniques for improving the mold warm-up time of the tyre curing process have received limited attention in previous studies, which have primarily focused on enhancing heat transfer in configurations and working conditions typical of heat exchanger applications. The objective of this article is to enhance the condensation heat and mass transfer rate by geometrically modifying the internal flow path of the circumferential rectangular channel, resembling an external jacket cavity of a tyre cure press, using twisted tapes and extended surfaces. Four configurations are simulated: i) conventional circumferential rectangular channel, ii) channel with twisted tapes, iii) channel with surface extension, and iv) channel with both twisted tapes and surface extension. The heat transfer performance of all the modified channel configurations derived from the simulation results is assessed in comparison to the conventional channel. The channel, featuring a design that includes five twisted tapes with a twist ratio of 8 and 54 extended surfaces, records a substantial 36.85% rise in heat flux along with a notable 24% increment in the condensation mass flux. The simulation results are validated using an in-house experimental study. The tyre cure press in the experimental setup is modified to align with the design proposed by the simulation results, and a warm-up study is conducted. The warm-up time for the modified mold to attain the desired temperature is reduced by 33%, with no accompanying increase in steam consumption, compared to the warm-up time for the conventional mold. This significant reduction in mold warm-up time holds immense potential in enhancing the productivity rate, an ongoing requirement of the automobile industry.

An experimental investigation of nucleate boiling performance for low-GWP azeotropic refrigerant R513A on a smooth tube

In the present study, an experimental investigation is conducted to study the nucleate boiling characteristics on a smooth tube having a low-GWP azeotropic refrigerant, R513A. The heat flux ranges from 5 to 90 kW/m2, and saturation temperatures are 0, 6, 10, 16, and 22 °C, respectively. The performance of R513A is compared against that of R134a. The performance is in terms of heat transfer coefficient (HTC). HTCs normally increased with the rise of heat flux and saturation temperature. However, the HTCs of R513A exceed those of R134a by approximately 20–25 %. Additionally, comparative analysis of the test data and some existing correlations are made. The compared correlations include three correlations based on reduced pressure (Cooper, Ribatski and Jabardo, and Jung), as well as two correlations based on thermophysical properties. It is found that the Jung correlation gives the best overall predictive ability against the R513A data with a deviation ranging from 5 % to 10 %.

High flow rate electrospray cooling performance of water–ethanol mixtures

Electrospray (ES) is an effective approach for cooling high-power density devices, owing to its desirable features such as uniform droplet diameter, suppression of droplet rebound, and low power consumption. Despite its excellent thermal properties, water has not been extensively employed as a working fluid in ES setups due to difficulties in establishing a stable ES mode. In this study, ES cooling performance of high flow rate water–ethanol mixtures is investigated. The stable region of the cone-jet mode is identified by capturing the meniscus profile and detecting the corona discharge. The study investigates the effect of ES variables, including flow rate (20 mlhr −100 mlhr), applied voltage (5.30 KV − 12.15 KV), and working fluid including pure ethanol (xH2O= 0 %), mixture I (xH2O= 35 %), and mixture II (xH2O= 70 %), on the cooling performance within the stability region. The results show that raising the flow rate from 20 mlhr to 100 mlhr augments the onset voltage of the cone-jet mode in each fluid. In the case of pure ethanol and mixture I, the end voltage corresponding to the multi-jet mode increases with the rise in flow rate. However, As a result of the heightened surface tension, no multi-jet mode is observed for mixture II, and only the occurrence of corona discharge defines the end voltage. In the single-phase regime, the cooling behavior only depends on the flow rate, owing to the thick liquid layer formed on the heated surface. However, it has been revealed that when the mixture water volume fraction increases up to 70 %, the critical heat flux rise is approximately82 % (Q = 20 mlhr), 148 % (Q = 60 mlhr), and 167 % (Q = 100 mlhr), at the onset voltage and 98 % (Q = 20 mlhr), 131 % (Q = 60 mlhr), and 147 % (Q = 100 mlhr), at the end voltage due to the improved thermal properties of the fluid. Next, the effect of each ES variable on cooling performance is investigated separately. Based on the results, an increase in the flow rate from 40 mlhr to 80 mlhr leads to an approximately 70 % rise in the critical heat flux for both mixture I and mixture II. Additionally, increasing the mixture water volume fraction from 35 % to 70 % results in an approximately 29 % rise in the critical heat flux, attributed to the larger latent heat of mixture II. In the transition regime, mixture II demonstrates superior performance compared to mixture I by suppressing the rebounding effect and enhancing the cooling performance. This improvement is due to the rise in the image force caused by the increase in water volume fraction. However, this enhancement is more significant at the lower flow rate due to the shorter impact time.

Numerical simulation on flow boiling heat transfer of R1234ze(E)/R152a in a horizontal smooth tube

The understanding of two-phase flow and heat transfer characteristics of refrigerants in tubes is important for guiding the design and optimization of heat exchangers. Based on the volume of fluid (VOF) multiphase model, this paper established a numerical model of R1234ze(E)/R152a (mass ratio of 0.4/0.6) flow boiling heat transfer in a horizontal smooth copper tube with an inner diameter of 6 mm and a length of 900 mm. The distribution of vapor volume fraction is obtained, and the influence of mass flux, heat flux, saturation temperature, and vapor quality on heat transfer coefficient (HTC) are studied. Bubble flow, plug flow, stratified flow, and wavy flow can be observed during the whole process and the fluid temperature increases along the tube. Local and time-averaged heat transfer coefficients and temperature distribution along the axial direction were studied. And the results indicate that the HTC decreases first and then increases with the augmentation of mass flux while increasing with the rise of heat flux. In addition, the HTC rises along with saturation temperature and decreases along with vapor quality. The largest related standard deviation between the simulation value and the testing data is 6.31%. Thus, the numerical simulation has a high level of accuracy.

An Eulerian multiphase frost model based on heat transfer measurements

In this paper, a laminar numerical model is developed to predict frost formation over a horizontal cold flat surface. An Eulerian-Eulerian multiphase approach is followed to model humid air and ice phases separately. Frost accumulation on the cold surface is modeled with an empirical mass source term. Model constants were tuned in a systematic way using experimental heat flux and frost thickness data. A velocity dependent model constant is introduced into the mass source term. The heat flux rise observed experimentally at the initial stages of the frosting could be captured with the use of the velocity dependent model constant and the addition of this term considerably improved the accuracy of the frost model at the initial stages of frosting. It was also observed that the selected particle diameter for the solid ice phase has a considerable effect on the velocity profile over the frost layer. This requires tuning of the velocity dependent model constant parameters according to the selected ice particle diameter. The developed numerical model was tested with three different frost thermal conductivity models. Using the thermal conductivity of solid ice for the frost thermal conductivity resulted in the most accurate prediction at the early stages of the frost growth process indicating a rather column-wise vertical growth of ice crystals with very low lateral branching. However, the overprediction of the numerical heat flux with the thermal conductivity of solid ice points out a decrease in the thermal conductivity of the newly added frost layers indicating a more pronounced lateral branching of ice crystals within the frost layer. The effect of the diffusion coefficient of the water vapor in humid air on frosting is also investigated. An artificial increase in the diffusion coefficient improved the accuracy of the heat flux prediction of the model at the initial stages of frosting which might indicate an eddy-driven enhanced mixing in the boundary layer which might not be captured in the laminar flow model. Finally, the developed numerical model is also tested on another scenario with the surface temperature held at -30 °C. Detailed analysis of the numerical simulations showed a more porous frost layer with the surface temperature at -30 °C as compared to the frost porosity formed on the surface at -20 °C.

Energy, exergy and economic (3E) analysis of flat-plate solar collector using novel environmental friendly nanofluid

The use of solar energy is one of the most prominent strategies for addressing the present energy management challenges. Solar energy is used in numerous residential sectors through flat plate solar collectors. The thermal efficiency of flat plate solar collectors is improved when conventional heat transfer fluids are replaced with nanofluids because they offer superior thermo-physical properties to conventional heat transfer fluids. Concentrated chemicals are utilized in nanofluids' conventional synthesis techniques, which produce hazardous toxic bi-products. The present research investigates the effects of novel green covalently functionalized gallic acid-treated multiwall carbon nanotubes-water nanofluid on the performance of flat plate solar collectors. GAMWCNTs are highly stable in the base fluid, according to stability analysis techniques, including ultraviolet–visible spectroscopy and zeta potential. Experimental evaluation shows that the thermo-physical properties of nanofluid are better than those of base fluid deionized water. The energy, exergy and economic analysis are performed using 0.025%, 0.065% and 0.1% weight concentrations of GAMWCNT-water at varying mass flow rates 0.010, 0.0144, 0.0188 kg/s. The introduction of GAMWCNT nanofluid enhanced the thermal performance of flat plate solar collectors in terms of energy and exergy efficiency. There is an enhancement in efficiency with the rise in heat flux, mass flow rate and weight concentration, but a decline is seen as inlet temperature increases. As per experimental findings, the highest improvement in energy efficiency is 30.88% for a 0.1% weight concentration of GAMWCNT nanofluid at 0.0188 kg/s compared to the base fluid. The collector's exergy efficiency increases with the rise in weight concentration while it decreases with an increase in flow rate. The highest exergy efficiency is achieved at 0.1% GAMWCNT concentration and 0.010 kg/s mass flow rate. GAMWCNT nanofluids have higher values for friction factor compared to the base fluid. There is a small increment in relative pumping power with increasing weight concentration of nanofluid. Performance index values of more than 1 are achieved for all GAMWCNT concentrations. When the solar thermal collector is operated at 0.0188 kg/s and 0.1% weight concentration of GAMWCNT nanofluid, the highest size reduction, 27.59%, is achieved as compared to a flat plate solar collector with water as a heat transfer fluid.

Performance analysis of micro-fin tubes compared to smooth tubes as a heat transfer enhancement technique for flow condensation

• Entropy generation of R134a flow condensation inside micro-fin and straight tubes is studied and compared. • Geometrical parameters impact on entropy generation is investigated. • Flow conditions impact on entropy generation is analyzed. • Favorable geometrical and flow conditions at which the micro-fin tube has superior performance are identified. Heat transfer enhancement techniques are accompanied by pressure drop amplification, detrimentally affecting their performance; entropy generation analysis is an effective approach to assess heat transfer enhancement along with resulting pressure drop. Current study investigates and compares the performance of micro-fin (as a passive enhancement technique) and smooth tubes during flow condensation (for R134a refrigerant) through conducting entropy generation analysis. First, the impact of geometrical and operating variables on pressure losses and heat transfer contributions to entropy generation and total generated entropy inside both types of tubes is examined. Then, the conditions at which the application of micro-fin tubes in lieu of smooth ones is justifiable and of superior performance are identified utilizing entropy generation number. The simulation results indicate that entropy generation enhances in the micro-fin tubes as tube diameter, mass velocity, vapor quality, and wall heat flux rise, and saturation temperature declines. The same is observed in the smooth tube except for the mass velocity; an increase in this parameter leads to a decreasing-increasing trend in entropy generation. Moreover, the entropy generation number results indicate that applying micro-fin tubes rather than smooth ones is justifiable, i.e., has better performance, at lower mass velocities and vapor qualities, but higher saturation temperatures and wall heat fluxes.

Flow boiling performance and bubble behaviors of non-closed droplet micro pin-fin arrays

Bubble characteristics are closely related to the thermal-hydraulic performance of working fluid in micro scale. This study aimed to investigate the flow boiling performance of non-closed droplet micro pin-fin arrays and bubbles behaviors in three different regions around micro pin-fins. Deionized water was selected as coolant, circulating under heat flux in the range of 2.51–144.64 W/cm2, of which high-speed images were attained, showing the flow field behaviors of coolant and the growth characteristics of bubbles at nucleate boiling. The experiments were carried out as mass flow rate ranging from 53.41 to 266.92 kg/(m2·s), and comparative analysis on flow and thermal performance were conducted. The visualization results presented that the flow boiling behavior of coolant could be divided into four states: single-phase flow, sub-cooled boiling, nucleate boiling and transition boiling state. The waiting time and growth time of bubbles in region III were smaller than that in region II, which was shown closely related to the larger flow velocity of coolant. The pressure drop of non-closed droplet micro pin-fin arrays underwent three stages as the rise of heat flux. The heat transfer coefficient could be ascended by increasing the mass flow rate during flow boiling.

Thermal-hydraulic behavior of lotus like structured rGO-ZnO composite dispersed hybrid nanofluid in mini channel heat sink

An experimental investigation in mini channel heat sink has been performed to assess the thermal-hydraulic behavior of composite dispersed hybrid nanofluid. Reduced graphene oxide-Zinc oxide (rGO-ZnO) nanocomposite particles have been synthesized and dispersed in water to prepare hybrid nanofluid with 0.01% volume concentration. Effects of different Reynolds number (50–600), volumetric flow rate (0.1–0.5 lpm), heat flux (33.33–66.67 W/cm2), channel aspect ratio (2.5–5; corresponding hydraulic diameter: 1.14–1.33 mm) on heat transfer, pressure drop and their relative performance characteristics are examined in details. Reduction in channel aspect ratio from 5 to 2.5 (hydraulic diameter from 1.33 to 1.11 mm) enhances the convective heat transfer coefficient by 47.2% at a flow rate of 0.5 lpm. The heat transfer coefficient significantly increases by about 17.41%, with rise in heat flux from 33.33 W/cm2 to 66.67 W/cm2. The effect of heat flux on the friction factor is not significant throughout the Reynolds number range. Performance evaluation criteria has a decreasing-increasing-decreasing trend with flow rate for all channel aspect ratios and heat fluxes. An optimal flow rate has been observed for the figure of merit at all the combinations of heat flux and channel aspect ratio. The figure of merit has a maximum value of 1.39 for a channel aspect ratio of 3.75 (i.e., hydraulic diameter of 1.26 mm) and heat flux of 66.67 W/cm2.

Emergency cooling of superheated surfaces by nanofluids additives in the event of a water boiling crisis

In order to prevent accidental overheating in metallurgy, electrical engineering, power engineering, nuclear power plants (NPP) etc., we have developed a series of nanofluids (NFs) which are stable under high temperature, boiling and radiation conditions. In our work, a new AlSi-7 nanofluid based on natural aluminosilicates with unique thermal behavior was developed for the first time. The boiling curve, covering all modes including transient and film boiling and burnout heat flux up to 3.7 MW/m2 is presented. The possibility of using this NF for emergency cooling of a superheated heat exchange surface was studied. For this purpose, the automatic unit for simultaneous recording of NFs boiling curves and the changes of the main heat transfer parameters (such as heat flux, heat transfer coefficient and temperature of the heating surface) in real time conditions under a constant rise of heat flux are presented. These NFs consisted of metals oxides, natural aluminosilicates and carbon nanomaterials (including nano-diamonds, multi-walled carbon nanotubes (CNTs) and thermo-expanded graphite (TEG)). Results showed the possibility of increasing the heat transfer coefficient (HTC) and critical heat flux (CHF) by 1.5–3.0 times compared with those of distilled water. By adding nanofluids to boiling water in the event of a boiling crisis, emergency cooling of an overheated heat transfer surface resulted in a sharp decrease in temperature of the heating surface from 700 to 125 °C without reducing the heat load has been proven.

Experimental study on shell side heat transfer characteristics of two-phase propane flow condensation for vertical helically baffled shell-and-tube exchanger

Vertical helically baffled shell-and-tube heat exchangers (HBHXs) occupy smaller ground areas compared to horizontal HBHXs and have been used as propane condensers in liquefied natural gas (LNG) plants. The heat transfer characteristics of propane in the shell side of vertical HBHX were investigated in the present study. The flow patterns and heat transfer coefficients in the shell side of a vertical HBHX test sample were obtained, and the results were compared with the existing data of the horizontal HBHX. The experimental conditions cover mass flux of 20–40 kg m−2 s−1, heat flux of 3.0∼7.0 kW m−1, and vapor quality from 0.2 to superheated. It is found that the variation trend of propane heat transfer coefficient with the increase of vapor quality in the vertical HBHX shell side is similar to that of the horizontal one. However, opposite to the horizontal HBHX, the heat transfer coefficient increases with the rise of heat flux for two-phase propane in the vertical HBHX shell side, probably arising from the influence of convection heat transfer in the liquid film. The heat transfer coefficient of two-phase propane in the vertical HBHX is 15–55% lower than that in the horizontal one, due to the thicker liquid film on tubes in the vertical HBHX. A correlation was developed to predict the heat transfer coefficient, and it agrees with 93% of experimental data within a deviation of ±20%.

Experimental study on onset of nucleate boiling and flow boiling heat transfer in a 5 × 5 rod bundle at low flow rate

Onset of nucleate boiling and flow boiling heat transfer are experimentally investigated in a 5 × 5 rod bundle at low flow rate under the pressure of 6 MPa. The experimental mass flux ranges from 25 to 200 kg/m2 s, heat flux varies from 25 to 75 kW/m2, the maximum vapour quality is 0.45. Effects of parameters on ONB and flow boiling are examined. The wall superheating at ONB point increases with the rise of heat flux while decreases with the increase of mass flux. The effects of mass flux and quality on flow boiling heat transfer coefficients are not distinct. For both subcooled and saturated flow boiling in the 5 × 5 rod bundle, the effect of heat flux on heat transfer is more remarkable at high flow rate than at low flow rate. Comparisons between predictions by existing empirical correlations and the experimental data are conducted. Basu’s correlation could predict the wall superheating at ONB point within a deviation less than 30% under conditions of higher mass flux (>150 kg/m2 s). As for the comparisons in flow boiling regions, predictions by Chen’s correlation agree well with the experimental data in both subcooled and saturated regions, except under conditions of low mass flux with low heat flux. Most of the correlations for boiling heat transfer and ONB are derived from the experimental data obtained at high flow rate, however, current experiments indicate that these correlations cannot be directly used under low flow rate conditions.

Experimental study of flow boiling instabilities in horizontal tubes under gas–liquid stratification condition: Effects of heat flux and inlet subcooling degree

A gas–liquid stratified flow boiling was performed in a 1530 mm long copper tube with an inner diameter of 12 mm under forced convection condition. Four different heat fluxes (q = 15.43 kW/m2, 18.52 kW/m2, 21.60 kW/m2 and 24.69 kW/m2) and inlet subcooled temperatures (ΔT​sub = 10 °C, 15 °C, 20 °C and 25 °C) were applied to investigate the characteristics of flow boiling instabilities, respectively. The results show that the instability features were closely related to the heat flux and inlet subcooling degree. With the increase of heat flux or decrease of inlet subcooling degree, two types of instability oscillation, DWO (density wave oscillation) and PDO (pressure drop oscillation) were identified. Moreover, an additional enhancement of heat transfer coefficient was found when the transition of the oscillation type from DWO to PDO happened. In the meantime, the frequency and amplitude of the PDO type oscillation were also increased with the rise of heat flux or the drop of subcooling. The intensification of fluctuation in PDO, however, has deteriorated the flow boiling heat transfer.

A study of heat transfer scaling of supercritical pressure water in horizontal tubes

Understanding the role of the heat transfer scaling in supercritical utilities can help us obtain a better design for supercritical single phase thermosiphons. In this paper, heat transfer scaling analysis of supercritical pressure water in horizontal tubes with differential inside diameters are experimentally investigated. The operating conditions included mass flux of 100–600kg/m2s and heat flux of up to 400kW/m2. The uneven circumferential temperature distributions are discussed firstly over a broad range of heat fluxes. A significant temperature discrepancy exists between the top and bottom surface in each pipe, deteriorated heat transfer occurs on the top surface but enhanced heat transfer happens in the bottom region. With the rise of heat flux, more severe deterioration appears on the higher domain. Then a qualitatively comparison on the heat transfer characteristic of supercritical flow in different pipes are implemented. Evidently, with the increase of tube diameter, the inner-wall temperature peak is much pronounced and the degree of deterioration at the top surface much severe due to the intensification of buoyancy force. The heat transfer mechanism of horizontal pipes is further discussed by comparing the effect of buoyancy and thermal acceleration. With the augment of diameter, free convection gradually intensifies due to its steeper density variation and expanded flow space, which leads to larger temperature deviation between the bottom and top region along the circumferential direction. On the contrary, thermal acceleration plays a relatively minor role in deteriorated heat transfer. Considering the scaling effect in various pipes, two heat transfer correlations are proposed respectively in both the bottom and top region of the horizontal flows.

Convective Structure and Heat Transfer in Transient, Evaporating Films

The fluid mechanics and heat transfer of evaporating planar films under unsteady conditions were studied experimentally. The initially quiescent films were subjected to superheating by rapidly dropping the pressure, and the films evaporated into their own vapor only. The test fluids were various hydrocarbons with film thicknesses of 1–5 mm. Ultrasound ranging was used to measure changes in the film thickness, from which the heat flux at the liquid surface was determined. Double-pass schlieren imaging was employed to image the convection patterns in the film. Two distinct rises in the heat flux were observed. The initial rise in heat flux was associated with the transient superheating and was not accompanied by convective structure within the film. The second rise in heat flux was associated with a substantial evolution in the convective structure and the associated wavelength. The penetration depth of the convective structures in films undergoing transient evaporation was estimated based upon the wavelength-to-depth ratio versus Rayleigh number trends observed in quasi-steady experiments. The estimated penetration depth was less than the actual film depth, suggesting that the wall boundary condition was not important in transient, evaporating films at the onset of convection.

Numerical investigation on onset of significant void during water subcooled flow boiling

Accurate evaluation of the location of onset of significant void (OSV) is particularly important in predicting the void fraction profile in the subcooled flow boiling. In this study, a Computational Fluid Dynamics (CFD) model based on a wall heat flux partition algorithm is presented. Good consistency between simulation results and available experimental data for void fraction, liquid temperature, and wall temperature is demonstrated for a given case. To further evaluate the feasibility of the CFD model in predicting the OSV condition, more simulations are performed under various operational parameters. The simulation results show that an acceptable prediction is observed in the inlet and outlet sections for the void fraction in all the cases compared with experimental data. In addition, the relative enthalpy of the flow at OSV decreases with a rise in heat flux and with decrease in mass flux and with increase in pressure obviously, which coincide well with the experiments. Meanwhile, the change trends for the liquid subcooling at OSV of the simulation results and the results calculated by Saha and Zuber correlation are similar and the error is within −7.15 to 8.09K. Therefore, the CFD model can obtain a comparatively accurate liquid subcooling at OSV.

An experimental study of inclination on the boiling heat transfer characteristics of a micro-channel heat sink using HFE-7100

In this study, the effect of inclination on the two-phase convective boiling heat transfer characteristics of HFE-7100 is investigated experimentally in a multiport micro-channel having a hydraulic diameter of 440μm. The corresponding mass flux is 100 and 200kgm−2s−1, respectively. The inclinations include −90, −45, 0, 45, and 90° with respect to the horizon. Test results show that normally the heat transfer coefficient for upward arrangements is superior to those of downward arrangement at a supplied heat flux of 25kWm−2. Yet this is applicable for G=100 or 200kgm−2s−1. The heat transfer coefficients increase with the vapor quality and peak again at a vapor quality of 0.6, suggesting a controlled mechanism of convective boiling. With the rise of heat flux to 40kWm−2, it appears that nucleate boiling may impose appreciable effect on the heat transfer mechanism. For G=100kgm−2s−1, the heat transfer coefficients for all arrangements are insensitive to change of vapor quality (x<0.6), implicating that nucleate boiling is in control. Analogous results prevail for G=200kgm−2s−1. Six heat transfer correlations are used to predict experimental data in different orientation. It appears that the Kandlikar correlation shows the best predictive ability against the present data with an overall mean deviation of 29.04%

Investigation of the Dynamics of Nucleate Boiling in Subcooled Falling Liquid Films

This paper deals with the features of nucleation dynamics at boiling in falling water films at different subcooling, Reynolds number and heat fluxes. With the use of high-speed infrared and digital video the local parameters of nucleate boiling in falling liquid films such as: bubbles’ diameter before condensation, frequency of nucleation and temperature of onset of bubble appearance were received. Analysis of the experimental data showed that bubbles’ diameter before condensation has strong dependence on initial temperature and increases with the rise of heat flux. The main influence on nucleation frequency has the variation of heat flux density. At the same time the experimental data on nucleation frequency in falling water films are close to the frequency of nucleation at pool boiling. To identify the main features the comparison of received data on the local characteristics at boiling in subcooled falling liquid film with existing models for pool boiling was made

Thermal analysis of charring materials based on pyrolysis interface model

Charring thermal protection systems have been used to protect hypersonic vehicles from high heat loads. The pyrolysis of charring materials is a complicated physical and chemical phenomenon. Based on the pyrolysis interface model, a simulating approach for charring ablation has been designed in order to obtain one dimensional transient thermal behavior of homogeneous charring materials in reentry capsules. As the numerical results indicate, the pyrolysis rate and the surface temperature under a given heat flux rise abruptly in the beginning, then reach a plateau, but the temperature at the bottom rises very slowly to prevent the structural materials from being heated seriously. Pyrolysis mechanism can play an important role in thermal protection systems subjected to serious aerodynamic heat.

Thermal entry length in falling liquid films at high Reynolds numbers

A behavior of the length of thermal entry was studied experimentally in the vertically falling wavy liquid film at high Reynolds numbers with application of the field methods for thickness and temperature measurements. According to data, obtained by a high-speed IR camera, the zones, whose temperatures exceed the inlet temperature of the film, are formed at 7–10mm from upper edge of the heater in the interrivulet area between the crests of developed 3D synchronous waves even at low heat fluxes. It was determined that in the area of rivulets, formed on the crests of 3D synchronous waves, the length of thermal entry is higher, but it decreases drastically with a rise of heat flux. Analysis of data on the film thickness, obtained by the fluorescent method, has shown that between the synchronous 3D waves in spanwise direction the average film thickness is lower. Respectively, the values of local Reynolds numbers of the film and distances before the appearance of temperature nonuniformity decrease. In these regions the effect of thermocapillary forces is obvious even at relatively low heat fluxes, what finally leads to a considerable decrease in the length of thermal entry.