Conditions Of Free Convection Research Articles

Quantification of surface layer turbulence using sensible heat values from energy balance versus aerodynamic methods

Surface layer optical turbulence values in the form of the refractive index structure function C n 2 are often calculated from surface layer temperature, moisture, and wind characteristics and compared to measurements from sonic anemometers, differential temperature sensors, and imaging systems. A key derived component needed in the surface layer turbulence calculations is the sensible heat value. Typically, the sensible heat is calculated using the bulk aerodynamic method that assumes a certain surface roughness and a friction velocity that approximates the turbulence drag on temperature and moisture mixing from the change in the average surface layer vertical wind velocity. These assumptions/approximations generally only apply in free convection conditions. To obtain the sensible heat, a more robust method, which applies when free convection conditions are not occurring, is via an energy balance method such as the Bowen ratio method. The use of the Bowen ratio––the ratio of sensible heat flux to latent heat flux––allows a more direct assessment of the optical turbulence-driving surface layer sensible heat flux than do more traditional assessments of surface layer sensible heat flux. This study compares surface layer C n 2 values using sensible heat values from the bulk aerodynamic and energy balance methods to quantifications from sonic anemometers posted at different heights on a sensor tower. The research shows that the sensible heat obtained via the Bowen ratio method provides a simpler, more reliable, and more accurate way to calculate surface layer C n 2 values than what is required to make such calculations from bulk aerodynamic method-obtained sensible heat.

Large-scale coherent structure and canopy-level turbulence in the convective boundary layer over urban areas

Thermally unstable stratification alters the winds over urban areas and excites organized, large-scale motions (LSMs) in the atmospheric boundary layer (ABL). The current understanding of the interaction between urban turbulence and buoyancy-induced coherent structures is rather limited due to their multiscale, multiphysics nature. Nine sets of large-eddy simulation (LES) are performed to critically examine the influence of atmospheric instability on turbulent boundary layers (TBLs) interacting with idealized urban roughness elements. Varying the stratification from neutral to almost free convective conditions enables the coherent structures to transit from horizontal rolls to open cellular patterns. The aerodynamic roughness parameters tend to decrease in more unstable stratification, potentially a result of buoyancy effects that might enhance organized ejections and suppress energetic sweeps within the roughness sublayers (RSLs). Unexpectedly, roughness-induced sweeps dominate the vertical momentum fluxes within urban canopies even under the most convective condition. However, decoupling the momentum and heat transports in highly convective states results in a subtle influence of urban roughness on the vertical heat fluxes therein. The spatial distribution of canopy-level turbulent momentum fluxes is found to be strongly modulated by the overlying large-scale coherent structures. The modulation is enhanced with increasing instability. In addition, urban-like surfaces elevate the large-scale rolls and assist them in forming under a weaker stratification than in canonical settings.

SIMULATION OF FLOW AND HEAT EXCHANGE IN A HEAT ACCUMULATOR TANK

The article deals with the research of thermal energy storage tanks. It is proposed to use a «thermal core» to minimize the effects of thermal stratification and high thermal inertia. The thermal core consists of a binary tube placed along the central axis of the tank, filled with a paraffin mixture with a melting point of 45 to 65oC and a density of 0,880 to 0,915g/cm3 at 15oC. In the study the Fluent software package was used to model the temperature distribution in the tank under free convection conditions, the data was then converted to the Transient Thermal module of the ANSYS software package for further calculations of the unsteady temperature distribution in the thermal core. The study showed that a 1400-liter thermal energy storage device, heated for 1 hour by a heat-transfer fluid at 115oC, cools down to 50oC in 4 hours. The research also revealed the need to improve the tank design based on the analysis of the hydrodynamic structure of the flow in the tank, as evidenced by the trajectory distribution of free convective flows. The authors concluded that the use of a heat core, regardless of the type of paraffin used to form it, helps to reduce temperature stratification by height in the tank and that the type of paraffin used has no significant effect on the overall cooling of the tank. However, using ceresin as a core filler results in a slightly higher average tank temperature. Based on the results of the study, the time of complete cooling of the tank was determined by the non-uniform temperature field of all elements of the tank.

Design of near-ideal and omnidirectional selective solar absorber for high-temperature applications

Efficient absorption of solar radiation with minimal thermal losses is required for efficient solar-to-thermal energy conversion. For this purpose, spectrally selective and omnidirectional surfaces are required that have strong, ideally unity, absorption in the solar spectral window for all incident angles and low, ideally zero, emissivity in the spectral range of blackbody radiation at operating temperature. In this article, we propose theoretically and experimentally lithographic free broadband and omnidirectional solar absorber for high-temperature applications made in a dielectric-metal-dielectric-metal fashion. We have used chromium (Cr), titanium (Ti), and tungsten (W) as a metal layer in the designed absorber due to their high thermal stability at high temperatures and compared their performance. The power balance equation is used to calculate the steady-state temperature considering free and forced convection conditions. W-based absorber shows >90% absorption in the solar window below cutoff wavelength for blackbody working at 600 K with <10% emissivity in the thermal window. Finally, we fabricate the W-based sample and experimentally measure the temperature rise of 100 K under one sun condition. The designed absorber can be used for several solar-thermal applications including solar-steam to electricity generation, water purification, space heating, and solar-thermal energy storage.

Effects of external magnetic field arrangements on thermomagnetic convection in ferrofluid around heated Micro-wire

Thermomagnetic convective cooling of a heated micro-wire in quiescent ferrofluid is experimentally investigated under various non-uniform external magnetic fields produced by permanent magnets. In the past studies, in most of the cases for this geometry the external magnetic field is applied under forced convection conditions. This work provides the first published data set of heat transfer from a microwire in quiescent ferrofluid (free convection conditions) under various arrangements of external magnetic fields produced by permanent magnets. For the different arrangements, magnetic flux density and Kelvin body force density are calculated to aid discussions of the observed effects. The experiment tracked the transient temperature rise of a heated micro-wire, highlighting effects of applied magnetic field orientation, symmetry, and strength. The results showed that greater non-uniformity in radial magnetic field could enhance the cooling effects with up to 7% reduction in the heated wire temperature rise. A magnetic field aligned with the heated wire axis produced only a minor enhancement of 3% in heat transfer in contrast to application of radial magnetic fields. Surprisingly, a stronger external magnetic field did not always result in better heat transfer. Magnetophoresis of ferro-particles induced by the external magnetic field was found to be an important consideration when using permanent magnets for ferrofluid applications.

Enhanced behaviour of a passive thermoelectric generator with phase change heat exchangers and radiative cooling

Heat exchangers are essential to optimize the efficiency of Thermoelectric Generators (TEGs), and heat pipes without fans have proven to be an advantageous design as it maintains the characteristic robustness of thermoelectricity, low maintenance and lack of moving parts. However, the efficiency of these heat exchangers decreases under natural convection conditions, reducing their heat transfer capacity and thus thermoelectric power production. This work reports on a novel heat exchanger that combines for the first time, phase change and radiative cooling in a thermoelectric generator to improve its efficiency and increase the production of electrical energy, specially under natural convection. For this, two thermoelectric generators with heat-pipes on their cold sides have been tested: one with the radiative coating and the other without it. Their thermal resistances have been determined and the electric power output was compared under different working conditions, namely, natural convection and forced convection indoors and outdoors. The experimental tests show a clear reduction of the heat exchanger thermal resistance thanks to the radiative coating and consequently, an increase of electric production 8.3 % with outdoor wind velocities of 1 m/s, and up to 54.8 % under free convection conditions. The application of the radiative surface treatment is shown to result in a more stable electrical energy production, suppressing the drastic decrease in the generated electric power that occurs in thermoelectric generators when they work under free convection.

Experimental investigation of an alternative battery pack thermal management system

Placing the batteries in PCM (Phase Change Material) filled containers causes a thermal imbalance in the pack because the heat can be dissipated from the sides, but it is accumulated in the center. An alternative geometry is proposed and tested to solve this problem. The thermal performance of the proposed battery pack structure employing PCM has been experimentally studied in free convection conditions. The battery pack is formed by placing 12 test batteries in the same size of 26650 LiFePO4 in custom-designed aluminum (Al 6065) chamber. The single-piece chamber consists of three subchambers connected by straight, rectangular, and parallel fins. The cells are positioned in the 4S3P arrangement. Three different PCMs with different melting temperatures are utilized, and their effect on the battery pack temperature is sought under a stagnant air environment (21 °C). The pack has been tested during discharging at 2C (1.3 W), 3C (2.75 W), and 5C (7.83 W). The heat generation values are adopted from the literature. The maximum temperature in the cells and the maximum temperature deviation between the cells are monitored while the cells are being charged or discharged for different test procedures. Instead of placing PCM and cells in a single case like in studies in the literature, a tailor-made finned geometry helped provide a more homogenous temperature in the cells thanks to the finned structure on the air side. In this way, the maximum temperature difference inside the battery pack is kept below 5 °C for all cases except the extreme case of cyclic 5C discharge/charge analysis with CrodaTherm47. The maximum temperature of the battery pack could be kept below the critical value (60 °C) only when CrodaTherm37 was employed for three consecutive cycles at a discharge rate of 5C/7.83 W as the fin temperatures could be kept at the optimum level, and the PCMs latent heat can be exploited the most.

Вільна конвекція — дослідження регулярного теплового режиму в різних шарах по висоті рідини

The rate of cooling (heating) of experimental liquids is investigated separately for five thermocouples located at different heights of the experimental probe. Research is carried out on an experimental stand in the system "environment I — body II", where "environment I" is water, and "body II" is the investigated liquid medium in a thin metal cylindrical shell under conditions of free convection. The rate of cooling (heating) is studied for test liquids: water, sunflower oil, distilled glycerin. The analysis of cooling (heating) processes during free convection of non-stationary heat exchange is carried out. This paper presents research carried out on an experimental stand, which consists of an external metal vessel, containing the environment with a temperature of t1, an internal cylindrical metal vessel containing an experimental liquid medium with a temperature of t2. Accordingly, probes with five thermocouples are placed in hot and cold liquids to take temperature values at a fixed time Non-stationary heat conduction processes occur during heating or cooling of bodies, substances, gaseous media and are accompanied by a change in enthalpy. Non-stationary and stationary processes of body heating due to the internal heat source, the electric heater, are considered in laboratory practice to determine the thermophysical properties of materials, bodies of substances and where cooling occurs due to the surrounding air. The analysis of the excess temperature, carried out separately for each thermocouple, placed along the height of the cylindrical volume, shows that the linearity of the dependence Ln ϑ = f(τ) is preserved, while the influence of free convection on the value of m is manifested. The value of m depends on the height of the liquid layer in which the temperature is measured.

Thermal performance prediction of a phase change material based heat-sink cooling system for a printed circuit board, using response surface method

The main objective of this experimental study is investigation of using phase change material (PCM) on the performance of a printed circuit board (PCB), as an electronic chipset. To this end, by a comparative study, the effects of using two types of PCMs (RT42 and RT35/CH), which have different melting points and latent heat of fusions, are examined on the cooling performance of the module under both free and forced convection. Experiments are conducted under two heat fluxes of 45 and 60 W. After selecting the appropriate PCM in terms of time to reach the critical temperature of 80 °C, the response surface method (RSM) is utilized to extract predictive models for considered responses. The predictive equations estimate the cooling performance of the PCM-based heat sink by considering PCM volume fraction and fan speed as two important independent design parameters in this study. By using RSM, the interaction effects of different factors on responses, and their grade of importance are presented. Under free convection condition, using PCM could significantly improve the usage time of the module and the enhancement ratio, especially in low heat flux of 45 W. Increasing the PCM volume fraction enhanced the usage time for both considered PCMs, although the RT42 showed better performance in all studied volume fractions compared to the RT35 HC. Comparison of two PCMs shows the superiority of using RT42 under both applied heat fluxes and operating condition. For the heat flux of 45 W, the highest thermal effectiveness is found for RT45 PCM/30 % (9.73) and for the heat flux of 60 it is found for RT45 PCM/90 % (3.67). According to the RSM, the PCM volume fraction significantly affects the cooling performance in comparison with the fan voltage.

Bejan’s thermal and mass flow visualization in micropolar fluid

Micropolar fluids are greatly utilized in biomedical applications as in biological; physical, chemical processes, lubrication systems, porous media and hydrodynamic-fluid problems and many others. An attempt is made to demonstrate the thermal and mass flow phenomena in micropolar fluids through the heat and mass functions concepts. The present physical model is devised based on the used geometry and free convection phenomena and gives the coupled nonlinear differential equations. Crank–Nicolson numerical scheme has been applied to disclose the silent flow and thermal features of micropolar fluid about a vertical cylinder. Physical behaviors of various parameters are discussed in terms of flow profiles and visualization patterns. Tabular solutions are presented to illustrate the importance of engineering quantities. Thermal and velocity contours are suppressed in the vicinity and enhanced away from the cylinder. Heatlines become closer to the heated surface, whereas masslines move far from the heated cylinder under free convection conditions for the rising Prandtl number. Guarantee of the obtained solutions are presented and shows an excellent agreement and this fact confirms the numerical accuracy.

Passive PV module cooling under free convection through vortex generators

High operating temperatures have a negative impact on photovoltaic (PV) modules. Hence, lowering the temperature of commercial crystalline silicon modules during their operation becomes increasingly interesting, as it increases the system's energy yield and prolongs the module's lifespan. In this work, we experimentally and numerically investigated a potentially cost-effective passive cooling method for PV modules, using vortex generators (VGs) optimised for free convection conditions (in the absence of wind). The VGs are attached to the rear surface of the module and arranged in an array. Infrared thermography was employed to assess the cooling ability of the VGs with different spacing. It is found that the thermally non-conductive VGs can reduce the operating temperatures of a module by up to 2 °C under free convection and by 3 °C with the use of conductive materials. To better understand the flow behaviour around the VGs, computational fluid dynamics simulations were undertaken, along with particle image velocimetry (PIV) measurements. The VG-induced mixing in the boundary layer is responsible for enhancing the convective flux on the rear module surface. We observed both that the formation of vortices is strongly dependent on the aerodynamic shape of the VG and that the placement of the VGs is crucial.

ГЕОДИНАМИЧЕСКАЯ СТРУКТУРА ТЕЧЕНИЙ В ВЕРХНЕЙ И НИЖНЕЙ МАНТИИ И ТЕПЛООБМЕН МЕЖДУ НИМИ

The object of the study is a two-layer geodynamic model of the Earth's mantle, including the upper and lower mantle. The subject of the study is the direction of flow at the top and bottom of the lower mantle. It is determined under the known geodynamic structure in the upper mantle, the known direction of the flow at its roof and in the presence of a lifting flow in the mid-ocean ridge area. The aim of the study is to reveal the character of interaction between the upper and lower mantle large-scale free flowing cells. At the upper-lower mantle boundary, two types of interaction of upper- and lower-mantle flows are possible: either unidirectional flows - cocurrent flow, or oppositely directed - counterflow. Horizontal large-scale currents at the 670 km boundary are a consequence of the horizontal temperature gradient under conditions of free convection. Heat transfer from the flow interface occurs under conditions of free convection, and the intensity of heat transfer at this boundary is determined by the vertical temperature gradient. The analysis of heat transfer in upper-mantle and lower-mantle flows at the 670 km boundary with cocurrent flow and counterflow is presented. The decrease in the average flow temperature for the lower mantle thermal boundary layer near the 670 km boundary with a counterflow at this boundary is estimated. The density and temperature differences in the mantle under the continent and the ocean, providing an excess of the surface level of the continent above the level of the ocean floor, equal to 7 km, are presented. Comparison of the estimated characteristic temperature difference with data on heat flows in the mantle under the ocean and the continent leads to the conclusion that there is a cocurrent flow of upper mantle and lower mantle flows at the 670 km boundary. The influence of the cocurrent flow at the 670 km boundary on the structure of large-scale mantle flows is analyzed. For two different situations of interaction of upper mantle flows under the ocean with the mantle under the continent, large-scale flows in the upper and lower mantle are presented

Нестаціонарний теплообмін у вертикальному циліндричному об’ємі, заповненому рідиною

The paper analyzes the conditions of convective heat transfer in the "limited volume" and in the "large volume". It is established that the process of heat exchange in the elements of the basic experimental setup in the system of experimental calculation method corresponds to heat exchange in a "large volume" under conditions of free convection. An experimental stand for the study of nonstationary heat exchange in the system "environment I ― body II", the main elements of which are two working cavities — the outer filled with water, volume V1, and the inner filled with the studied liquid medium volume V2, and V1 is larger V2 3 times. The results of experimental determination of heat transfer between the inner surface of a thin metal cylinder and the investigated liquid medium in a limited space for the system “environment I — body II” under the conditions of laminar regime of both the environment and the investigated liquid medium are presented. The heat transfer coefficient was investigated using the calculation-experimental method during heating and cooling of refined sunflower oil, distilled glycerin, sugar solution with a concentration of 50&nbsp;%, 60&nbsp;% under conditions of free convection. The experimental results are compared with the results of studies by well-known authors for "large volume" conditions. The conditions of the heat exchange process during the experiment at different directions of heat exchange in a "limited volume" (inside a thin-walled metal cylinder with the liquid under study) are described. It is substantiated that at the surface of a cylindrical metal wall in the process of heat exchange a thermal boundary layer is formed, within which the coolant temperature changes and heat transfer within the boundary layer occurs due to thermal conductivity. The criterion equations for determining the heat transfer coefficient during heating and cooling of the investigated liquid medium under free convection conditions are obtained.

ЧИСЛЕННАЯ МОДЕЛЬ ОГНЕНЕПРОНИЦАЕМОСТИ ЭЛЕМЕНТОВ ГАЗОТУРБИННЫХ ДВИГАТЕЛЕЙ ИЗ АЛЮМИНИЕВЫХ СПЛАВОВ

For the early stages of design, a numerical model of fire resistance and fire permeability of GTE elements made of alu-minum alloys is proposed. The numerical model is based on the solution of three-dimensional Navier-Stokes equations, closed by a two-parameter isotropic SST – turbulence model. We used an implicit pressure solver and a scheme of second-order accuracy in High Resolution space, which allows lowering the approximation order to the first in the areas of solution discontinuities and high gradients to increase the monotonicity and stability of the solution. Additionally, to achieve the solution stability, the High Speed Numerics solver option was used, activating the solver settings Gradient Factor Relaxation = 0.1 and Blend Factor Relaxation = 0.1. The mathematical model for the thermal-hydraulic calculation was set up in the formulation of the coupled heat transfer, taking into account radiation based on the discrete transfer model of the Participating Media. The influ-ence of the degree of blackness on the results of the numerical model is investigated and the most rational radiation model is chosen. The numerical model was validated according to the results of engineering tests on the fire permeability of the plate not cooled by the air flow using a kerosene burner NexGen of the bench Ts17-G3 of FAU P.I. Baranov TsIAM. The obtained results of calculations on fire permeability are comparable with the experimental data both quantitatively and qualitatively. The developed un-steady numerical model of fire permeability of flat plates of aluminum alloys under conditions of free convection can be used in the de-sign of the fan housing by selecting the most rational structural and layout solutions, such as the thickness of the housing, the presence of fire protection coating, cooling intensity, etc.

Вимушена конвекція — дослідження регулярного теплового режиму в різних шарах по висоті рідини

The rate of cooling (heating) of experimental liquids was studied separately for five thermocouples located at different heights of the experimental probe. Research has been carried out on an experimental stand in the system “environment I — body II”, where “environment I” is water, and “body II” is the investigated liquid medium in a thin metal cylindrical shell under conditions of free convection. The regular mode of cooling (heating) of the object is characterized by a change in the temperature field over time, which is described by an exponential dependence, while the relative cooling rate t of all points of the body remains a constant value, independent of coordinates and time. The theory of a regular thermal regime has been widely used to solve many practical problems: to determine the rate of cooling (heating) of a body, to study the thermophysical properties of materials, heat transfer and radiation coefficients, thermal resistance, etc. The advantages of this method are that the design of the device and the technique of conducting the experiment are simple, the accuracy of the obtained results is quite high, and the time of the experiment is short. Non-stationary thermal conductivity corresponds to a thermal regime unstable in time, created by this or that thermal action on the body or environment. Non-stationary thermal conductivity is characterized by the fact that the temperature changes not only from point to point, but also over time. Non-stationary thermal conductivity occurs during heating or cooling of bodies, as well as when starting or stopping heat exchange devices, power units, etc. The experimental results are presented on the study of the rate of cooling (heating) of experimental liquids separately for five thermocouples located at different heights of the experimental probe under conditions of forced convection

Local Similarity Theory of Convective Turbulent Layer Using “Spectral” Prandtl Mixing Length and Second Moment of Vertical Velocity

Abstract Approximations of the turbulent moments of the atmospheric convective boundary layer are constructed based on a variant of the local similarity theory. As the basic parameters of this theory, the second moment of vertical velocity and the “spectral” Prandtl mixing length are used. This specific choice of the basic parameters allows us to consider the coefficient of turbulent transfer and the dissipation of kinetic energy of the Prandtl turbulence theory as the forms of the local similarity. Therefore, the obtained approximations of the turbulent moments should be considered as natural complementation to the semiempirical turbulence theory. Moreover, within the atmospheric surface layer, the approximations of the new local similarity theory are identical to the relations of the Monin–Obukhov similarity theory (MOST). Therefore, the proposed approximations should be considered as a direct generalization of the MOST under free-convection conditions. The new approximations are compared with the relations of the known local similarity theories. The advantages and limitations of the new theory are discussed. The comparison of the approximations of the new local similarity theory with the field and laboratory experimental data indicates the high effectiveness of the proposed approach.

Revisiting the Surface Energy Imbalance

AbstractThe traditional surface energy balance (SEB) is investigated by applying the thermal energy balance based on total energy conservation. Based on total energy conservation, the thermal and the kinetic energy balances are connected through non‐hydrostatic energy transfer (QNH) as a result of the hydrostatic imbalance caused by potential energy changes from vertical density fluxes in response to mechanical and thermal forcing in the stratified atmosphere. Field observations show that both QNH and the surface energy imbalance (SEI) have the same diurnal variation; both are near zero under neutral conditions and increase with increasing atmospheric unstable stratification. Energy dissipation due to enhanced turbulent kinetic energy from QNH is estimated to be the largest energy consumption for potentially explaining the largest SEI under free convective conditions. The relatively large value of energy dissipation in the atmospheric surface layer is due to its exponential increase toward the surface and its increase with atmospheric instability. The other two energy transfers that are traditionally missed are related to water mass flowing through the open soil and air layers, but are relatively small in comparison with the energy dissipation. The new understanding of the energy transfers provides not only a potential explanation for the temporal and magnitude variations of the SEI but also the explanation for different heat/moisture transferring efficiencies between thermally and mechanically generated turbulent mixing. Understanding the SEB is important not only for solving the SEI but also for understanding energy conservation in the atmosphere.

Aggravated ozone pollution in the strong free convection boundary layer

Clarifying the relationship between meteorological factors and ozone can provide scientific support for ozone pollution prediction, but the effects of boundary layer meteorology, especially boundary layer height and turbulence, on ozone pollution are rarely studied. Here, ozone and its related meteorological factors were observed in summer in Shijiazhuang, a city with the most serious ozone pollution on the North China Plain. The forced and free convection boundary layers were classified using ground remote observations. After eliminating the forced convection condition, strong free convection conditions, exhibiting a high boundary layer height, high wind speed, strong turbulence and large-scale free convection velocity, were found to be beneficial for the aggravation of ozone pollution. Combined with the ozone profile detected by a tethered balloon, the ozone chemical budget was calculated using the differences in the column ozone concentrations between the morning and afternoon, and the results confirmed the impact of free convection intensity on ozone pollution. The change in ozone sensitivity from VOCs sensitivity to NOx sensitivity driven by strong free convection was the main reason for the deterioration of ozone pollution. This study clarified the impact of boundary layer meteorology on ozone and its sensitivity and has important practical significance for ozone pollution prevention and early warning.

Uncertainty in the Parameterization of Surface Fluxes under Unstable Conditions

AbstractIn this study, an attempt has been made to analyze the possible uncertainties in the parameterization of surface fluxes associated with the form of non-dimensional wind and temperature profile functions used in weather and climate models under convective conditions within the framework of Monin-Obukhov similarity theory (MOST). For this purpose, these functions, which are commonly known as similarity functions, are classified into four categories based on the resemblance in their functional behaviour. The bulk flux algorithm is used for the estimation of transfer coefficients of momentum and heat using four different classes of similarity functions. Uncertainty in the estimated values of fluxes is presented in the form of deviation in the predicted values of momentum and heat transfer coefficients and their variation with the Monin-Obukhov stability parameter. The analysis suggests that a large deviation in the values of estimated fluxes might occur if different forms of similarity functions are utilized for the estimation of surface fluxes. Recommendations are made for the form of similarity function for momentum based on the analysis of one year-long turbulence observations over an Indian region. The study suggests that there is a distinct need to carry out a careful analysis of turbulence data in free convective conditions for determining a consistent functional form of the similarity functions to be utilized in the atmospheric models universally.

An improved heat transfer model for steam-air mixture condensation with the curvature effect on vertical tubes

The condensation heat transfer on the external surface of a vertical slender tube in the steam-air mixture environment was investigated. The condensation heat transfer was analyzed using various open experimental datasets. A new model for the steam condensation with noncondensable gases applicable to the tubular configuration was developed. The proposed model is based on the analogy between heat and mass transfer; in particular, the correlation incorporates the effects of tube curvature under turbulent free convection conditions together with the wavy film effect. In addition, the film-vapor interfacial temperature was used to calculate the heat transfer coefficient and various physical properties, through an iterative technique instead of the wall temperature, which is commonly used in experimental correlations for convenience. Thereafter, the effects of three correction factors were individually investigated. The wavy film effect considerably changed the condensation heat transfer, about 20% on average. The curvature effect was also found to make a meaningful effect according to the convection conditions. And, the adoption of interfacial temperature was found to make a maximum 7% adjustment, but it would make a more significant effect in high heat transfer conditions. Overall, the proposed model predicted the experimental datasets reasonably well, i.e., within ±30% accuracy.