Conditions Of Free Convection Research Articles

Theoretical analysis of coal stockpile self-heating

This paper theoretically analyses the problem of coal stockpile self-heating under free convection (no wind) conditions. Scale analysis has been used to drive expressions for the stockpile temperature and inflection point (when the stockpile is completely dry) as functions of the key parameters including the coal type, moisture content, and particle diameter as well as the ambient air temperature. Theoretical predictions are then successfully compared with experimental data and CFD simulations results available in the open literature.

Estimation of local mass burning rates for steady laminar boundary layer diffusion flames

A thorough numerical and experimental investigation of laminar boundary-layer diffusion flames established over the surface of a condensed fuel is presented. By extension of the Reynold’s Analogy, it is hypothesized that the non-dimensional temperature gradient at the surface of a condensed fuel is related to the local mass-burning rate through some constant of proportionality. First, this proportionality is tested by using a validated numerical model for a steady flame established over a condensed fuel surface, under free and forced convective conditions. Second, the relationship is tested by conducting experiments in a free-convective environment (vertical wall) using methanol and ethanol as liquid fuels and PMMA as a solid fuel, where a detailed temperature profile is mapped during steady burning using fine-wire thermocouples mounted to a precision two-axis traverse mechanism. The results from the present study suggests that there is indeed a unique correlation between the mass burning rates of liquid/solid fuels and the temperature gradients at the fuel surface. The correlating factor depends upon the Spalding mass transfer number and gas-phase thermo-physical properties and works in the prediction of both integrated as well as local variations of the mass burning rate as a function of non-dimensional temperature gradient. Additional results from precise measurements of the thermal field are also presented.

Free and forced convection on the outer surface of vertical longitudinally finned tubes

The paper presents the results of experimental studies of the heat transfer process on the outer surface of longitudinally finned tubes. Experimental values of heat transfer coefficients under free and forced convection conditions were calculated for low air flow velocities. Two air flow patterns were analysed, i.e. airflow parallel and perpendicular to the axis of the pipe. The mean values of heat transfer coefficients obtained during the study amounted to 2–8.5 (Wm−2K−1) under free convection conditions, 4.5–19(Wm−2K−1) for forced transverse flow and 4–11(Wm−2K−1) for forced convection along the axis of the tube. Experimental values of heat transfer coefficients for transverse flow around the tube were 22% higher than heat transfer coefficients for airflow along the axis of the tube. The paper lists the dimensionless relationships which can be used to calculate the values of the heat transfer coefficient under free and mixed convection conditions, with transverse and longitudinal air flow around the outer surface of vertical longitudinally finned tubes.

Application of the windowed-Fourier-transform-based fringe analysis technique for investigating temperature and concentration fields in fluids.

The present work is concerned with the development and application of a novel fringe analysis technique based on the principles of the windowed-Fourier-transform (WFT) for the determination of temperature and concentration fields from interferometric images for a range of heat and mass transfer applications. Based on the extent of the noise level associated with the experimental data, the technique has been coupled with two different phase unwrapping methods: the Itoh algorithm and the quality guided phase unwrapping technique for phase extraction. In order to generate the experimental data, a range of experiments have been carried out which include cooling of a vertical flat plate in free convection conditions, combustion of mono-propellant flames, and growth of organic as well as inorganic crystals from their aqueous solutions. The flat plate and combustion experiments are modeled as heat transfer applications wherein the interest is to determine the whole-field temperature distribution. Aqueous-solution-based crystal growth experiments are performed to simulate the mass transfer phenomena and the interest is to determine the two-dimensional solute concentration field around the growing crystal. A Mach-Zehnder interferometer has been employed to record the path-integrated quantity of interest (temperature and/or concentration) in the form of interferometric images in the experiments. The potential of the WFT method has also been demonstrated on numerically simulated phase data for varying noise levels, and the accuracy in phase extraction have been quantified in terms of the root mean square errors. Three levels of noise, i.e., 0%, 10%, and 20% have been considered. Results of the present study show that the WFT technique allows an accurate extraction of phase values that can subsequently be converted into two-dimensional temperature and/or concentration distribution fields. Moreover, since WFT is a local processing technique, speckle patterns and the inherent noise in the interferometric data do not affect the resultant phase values. Brief comparisons of the accuracy of the WFT with other standard techniques such as conventional Fourier-filtering methods are also presented.

Surface protection by shielding from a hot source under conditions of free convection

In this paper, one way of heat protection from hot elements of the ground-based power-driven high-temperature gas turbine plant case have been experimentally studied by using shields placed between the case and outer skin under free convection in air gaps. Using the experimental model setup, experiments aimed at studying the temperature state of all its elements, namely, hot and cold walls, shield, and air gaps under free convection have been conducted. In the course of these experiments, the temperature of hot wall and air gap thicknesses were varied. The plots obtained by thermometering all walls are presented. As a result, generalization of the data obtained makes it possible to augment the data base and use it in choosing an optimal version of heat protection.

Smoke visualization under heat transfer by free convection on horizontal cylinder surface

The method of visualizing a dynamic field in conditions of free convection by introducing a laminar smoke stream into the area which is vertically lower than the existing thermal and dynamic boundary layers has been proposed and examined in the paper. The suggested method is simple, obvious, cheap and fast enough, that allows advising it for applying in the study of free-convective heat transfer problems using air as a heat-transfer agent. As a result of the conducted research, a set of assumptions, giving the basis to calculations and methods of analyzing experiment results has been demonstrated. In particular, among them there can be singled out the following: the laminarity of a heat-transfer agent motion mode, the laminarity of a thermal tracing and boundaries of its sustainable occurrence over a hot surface, the growth of boundary layer thickness in the cylinder stern, symmetry of washing cylinders from different sides from the vertical plane of symmetry, and others. Juxtaposing the results with the data, represented in the literature, shows a high correspondence.

Cooling-Performance Estimation of the Composite-Graphite Heat Sink with Annular Conductive Base for LED Lamp

This work experimentally investigated the material effect of heat sinks of LED lamps on the heat transfer and illumination characteristics under the free-convection condition. The experimental results indicated that the heat-transfer capacity of the pure aluminum-alloy heat sink was better than those of the composite-graphite heat sinks. However, the cost and weight of the composite-graphite heat sinks were reduced much more by comparing with those of the pure aluminum-alloy heat sink. Besides, the illuminations of these test heat sinks were similar after 2000-hour lighting continuously. Therefore, the composite-graphite heat sink is useful for the cooling of the LED lamp and the electric equipment. It has large potential in the relevant markets.

Experimental investigation of hydrodynamics and heat transfer in conditions of free convection

В работе представлено обобщение результатов экспериментальной работы по исследованию теплообмена и гидродинамики на поверхности горизонтального цилиндра в условиях свободной конвекции. Визуализация динамического слоя, который образуется вокруг нагретого цилиндра, позволяет наглядно подтвердить подходы, которые использовано при анализе результатов.

ЕкспЕримЕнтальнЕ дослідженнЯ теплообмінУ В умоваХ вільноЇ конвекціЇ нА поверхнІ гОризОнтальнОгО циліндрА

Despite the widespread and long-term investigation of heat transfer and hydrodynamics of a single horizontal cylinder in a large volume in conditions of free convection, both single approach to determining the intensity of heat transfer and final dependences of calculations are absent in the literature. The paper gives the results of the experimental study of heat transfer of the single horizontal cylin­der in conditions of free convection in the large volume, in the range of Rayleigh numbers: 9,1 · 103<Ra<1,7·105. The conclusion is drawn on the possibility of approximating the experimental results on one of the proposed in the literature dependences. In addition, the tem­perature distribution of the heat transfer agent around the cylinder was investigated. It was shown that the temperature of the medium changes sharply in the wall area and is almost unchanged in the rest of the volume. The temperature distribution is given in dimensionless coordinates, which correspond to both the most evident and the pro­posed in the literature. The experimental results are well coordinated with the results of mathematical modeling, presented in the literature. At the same time, comparing the results of measurements with the proposed in the literature analytical dependences of the dimension­less temperature distribution in the boundary layer, the conclusion is drawn on an underestimation of the rate of temperature change. To describe more accurately the change of the rate in the wall layer of the midship section is possible by the power dependence, with the indica­tor 7, not 2 - 4 as it is suggested in the literature.

Buoyancy effects on the integral lengthscales and mean velocity profile in atmospheric surface layer flows

Within the diabatic atmospheric surface layer (ASL) under quasi-stationary and horizontal homogeneous conditions, the mean velocity profile deviates from its conventional logarithmic shape by a height-dependent universal stability correction function ϕm(ζ) that varies with the stability parameter ζ. The ζ parameter measures the relative importance of mechanical to buoyant production or destruction of turbulent kinetic energy (TKE) within the ASL. A link between ϕm(ζ) and the spectrum of turbulence in the ASL was recently proposed by Katul et al. [“Mean velocity profile in a sheared and thermally stratified atmospheric boundary layer,” Phys. Rev. Lett. 107, 268502 (2011)]. By accounting for the stability-dependence of TKE production, Katul et al. were able to recover scalings for ϕm with the anticipated power-law exponents for free convective, slightly unstable, and stable conditions. To obtain coefficients for the ϕm(ζ) curve in good agreement with empirical formulas, they introduced a correction for the variation of the integral lengthscale of vertical velocity with ζ estimated from the Kansas experiment. In the current work, the link between the coefficients in empirical curves for ϕm(ζ) and stability-dependent properties of turbulence in the ASL, including the variation with ζ of the integral lengthscale and the anisotropy of momentum transporting eddies is investigated using data from the Advection Horizontal Array Turbulence Study. The theoretical framework presented by Katul et al. is revised to account explicitly for these effects. It is found that the coefficients in the ϕm(ζ) curve for unstable and near-neutral conditions can be explained by accounting for the stability-dependence of the integral lengthscale and anisotropy of momentum-transporting eddies; however, an explanation for the observed ϕm(ζ) curve for stable conditions remains elusive. The effect of buoyancy on the horizontal and vertical integral lengthscales is also analyzed in detail.

A method for estimating the sensible heat flux in the inertial sub-layer from high-frequency air temperature and averaged gradient measurements

It is shown that the sensible heat flux, H, can be estimated without measurements of wind speed. Over grass, H was estimated deploying three fine-wire thermocouples in the inertial sub-layer. Two thermocouples are required to measure the mean gradient and the other is required to measure the air temperature at high frequency to determine the mean ramp dimensions. Regardless of the stability conditions, excluding those cases where the gradients fall within the measurement error, the H estimates were close to the measured using the Eddy Covariance method, HEC. The cases where the method is not applicable (i.e., when the gradient is small), are near-neutral and under free convection conditions. In near-neutral cases, the air temperature trace does not show clear ramps and, therefore, H is close to zero. Under convection, H can be estimated using the free convection limit approach. It is shown that the new method can be used to calibrate the earlier surface renewal method which requires only one thermocouple to estimate H. Regardless of the stability cases, the earlier SR method calibrated against the EC method or against the new SR method performed close (the root mean square error, RMSE, were RMSE<1Wm−2).The new SR method was consistently best correlated (0.78≤R2≤0.90, where the highest and lowest R2 correspond to stable and unstable cases with positive net radiation, respectively) than the earlier SR method (0.71≤R2≤0.74) to HEC. The RMSE, however, for the new and the earlier SR method were comparable, in general, RMSE≤25Wm−2. It is concluded that for field applications the implementation of the new and the earlier SR method is an affordable and reliable alternative to the EC method to obtain long series of half-hourly H.

SIMULATION OF UREA PRILLING PROCESS: AN INDUSTRIAL CASE STUDY

A local industrial urea prilling tower with a rectangular cross-sectional area under the operating conditions of free convection was mathematically modeled. In this model the prilling process has been simulated by simultaneous solution of the continuity, hydrodynamic, and thermal equations. Temperature distributions of the particles and the cooling air along the height of the tower were calculated from the model. The air temperature profile predicted by the model was compared with the measured air temperature along the tower. The data predicted by the model were almost in agreement with the plant data, indicating the validity of the model. The result of this study suggests that an increase in heat transfer from the particles using installation of induced fans can reduce the temperature of produced prills and hence reduce caking tendency of the prilled urea.

Heat Transfer in Boiling of Binary Mixtures under Free-Convection Conditions

A study has been made of the heat transfer in boiling of binary mixtures of refrigerant–refrigerant-type homogeneous liquids completely soluble in each other under free-convection conditions with allowance for the effects of diffusion and evaporation of the highly volatile component from a mixture. These effects are responsible for the complex dependence of the heat transfer on the concentration of the mixture at a constant heat flux. A criterial dependence satisfactorily describing the results of experimental investigations has been proposed.

Investigation of vapor film motion regularities at boiling liquids

The experimental investigation of the saturated Freon-113 and distilled water film boiling on spheres with different diameters at atmospheric pressure under conditions of free convection is executed. With high-speed video average thickness and cumulative distribution function of vapor film as a function of the angle was measured. It was found that with increasing the angle the average thickness of vapor film can change by different laws depending on diameter of the sphere and the temperature difference. It was found also that the increase in the average vapor film thickness with increasing angle is more connected with the increase of large components of cumulative distribution function. It also noted the presence of quasi-periodic pulsations of the vapor film thickness in the lower part, which eventually largely determine the behavior of the interface at large angles.

Hans A. Panofsky’s Integral Similarity Function—At Fifty

Fifty years ago, Hans A. Panofsky published a paper entitled Determination of stress from wind and temperature measurements. In his famous paper, he presented a new profile function for the mean horizontal wind speed under the condition of diabatic stratification that includes his integral similarity function. With his integral similarity function, he opened the door for Monin-Obukhov scaling in a wide range of micrometeorological and microclimatological applications. In a historic survey ranging from the sixties of the past century down to the present days, we present integral similarity functions for momentum, sensible heat, and water vapor for both unstable and stable stratification, where on the one hand free convection condition and on the other hand strongly stable stratification are addressed.

Characterization of Heat Dissipation From a Microprocessor Chip Using Digital Interferometry

Electronic circuit boards of computers consist of components which generate substantial amounts of heat during their operation. Among these, the microprocessor chip is one of the major sources of heat generation. In high-performance processor chips, the rates of heat generation are also correspondingly high. The performance of the chips themselves, as well as that of the neighboring electronic components are affected by the temperature levels attained, which in turn depend on the heat dissipation capability. Effective thermal management of microprocessor chips, following precise thermal characterization, is essential to avoid their untimely failure due to overheating. The selection of an appropriate cooling methodology and the design of an efficient cooling system for the microprocessor chip essentially require a clear knowledge of the heat dissipation rates during its operation. A comprehensive database of heat dissipation from microprocessor chips as a function of the activity level, based on actual measurements, is rare to find in the literature. A major reason for this is that conventional intrusive measurements on and around the chip are quite insufficient, as they will alter the performance, while the system is under operation. To overcome this, a nonintrusive digital interferometric measurement has been devised in this paper for obtaining the in situ temperature distributions and the heat dissipation patterns around the microprocessor chip, while it is under operation. The interferometric method utilizes interference of coherent laser beams to make measurements in the thermal field around the heat-dissipating component. An Intel Pentium 4 microprocessor with its integrated heat sink was used for experimentation. The computational load on the processor was varied utilizing a software package, thus varying the heat generation rates, and the base temperature of the fins in the heat sink attached to the microprocessor. Interferometric measurements were made to obtain the temperature distribution in the medium, and the patterns of heat dissipation from the fins' surfaces. Indicators of the heat transfer behavior, namely, the local heat flux values and the heat transfer coefficients were obtained from the experiments, using digital image processing techniques. Experiments were conducted at various fin-base temperatures, under free and forced convection conditions, to characterize the effect through a database of the heat dissipation level. In essence, a nonintrusive temperature measurement method coupled with digital image processing techniques has been successfully used for convective heat transfer studies on the microchip while the microchip is under operation.

Areal-averaged trace gas emission rates from long-range open-path measurements in stable boundary layer conditions

Abstract. Measurements of land-surface emission rates of greenhouse and other gases at large spatial scales (10 000 m2) are needed to assess the spatial distribution of emissions. This can be readily done using spatial-integrating micro-meteorological methods like flux-gradient methods which were evaluated for determining land-surface emission rates of trace gases under stable boundary layers. Non-intrusive path-integrating measurements are utilized. Successful application of a flux-gradient method requires confidence in the gradients of trace gas concentration and wind, and in the applicability of boundary-layer turbulence theory; consequently the procedures to qualify measurements that can be used to determine the flux is critical. While there is relatively high confidence in flux measurements made under unstable atmospheres with mean winds greater than 1 m s−1, there is greater uncertainty in flux measurements made under free convective or stable conditions. The study of N2O emissions of flat grassland and NH3 emissions from a cattle lagoon involves quality-assured determinations of fluxes under low wind, stable or night-time atmospheric conditions when the continuous "steady-state" turbulence of the surface boundary layer breaks down and the layer has intermittent turbulence. Results indicate that following the Monin-Obukhov similarity theory (MOST) flux-gradient methods that assume a log-linear profile of the wind speed and concentration gradient incorrectly determine vertical profiles and thus flux in the stable boundary layer. An alternative approach is considered on the basis of turbulent diffusivity, i.e. the measured friction velocity as well as height gradients of horizontal wind speeds and concentrations without MOST correction for stability. It is shown that this is the most accurate of the flux-gradient methods under stable conditions.

Evaluation of evaporation estimation methods for a covered reservoir in a semi-arid climate (south-eastern Spain)

Summary The main purpose of this study was to evaluate different methods of evaporation estimation for covered water reservoirs. A reservoir equipped with a suspended cover was fully monitored to register the evaporation rate and microclimate below the cover. The datasets were used to evaluate the performance of commonly used evaporation methods, namely energy budget, mass-transfer, combination (Penman and FAO-56 Penman–Monteith) and floating class-A pan. The mass-transfer formula based on the Sherwood number proposed for free convection conditions, which were observed to prevail below the cover, supplied reasonably good estimates of covered reservoir evaporation and it is a good option from a practical point of view, with low input data requirements. Detailed input data and modifications in the calculation of energy fluxes are required to get good evaporation estimations of covered surfaces with the energy budget and FAO-56 Penman–Monteith methods. Besides, some of the standard meteorological input data (such as wind speed at 2 m height) cannot be registered below the cover. Penman equation presented a poor performance related to the overestimation of the advective component for free convection conditions. The pan evaporation was found to be substantially higher than the reservoir evaporation, due to the particular characteristics of the tank, that increased surface temperature and hence evaporation rate. A simplified empirical mass-transfer formula was also proposed to estimate evaporation of covered water bodies from the only knowledge of the surface-to-air mixing ratio gradient.

Investigation into the Phase Change of Refrigerant in a Wire–and–Tube Consender of Refrigerator

During the passage of dhe refrigerant through the condenser of a refrigerator, it generally undergoes the processes of desuperheating, condensation and sub–cooling. One of dhe important criteria in condenser design is to locate the points where superheated refrigerant vapour changes to saturated vapour and subsequently from wet vapour to saturated liquid. The present paper discusses the analysis of wire–and–tube condenser under normal operating conditions of free convection using FEM where Galerkin’s weighted residual method is used to minimise the errors. Refrigerant R–134a is considered for the study and the condenser components are divided into several elements where the state of refrigerant is predicted. The effects of ambient temperature and mass flow rates of the refrigerant are also determined. This is used to find out the length of tube required for phase change for its initiation and completion. The methodology adopted is validated against dhe simulation data available in the literature for both sensible and latent heat transfer. The analysis also leads to the information about dhe adequacy of the number of tubes for complete condensation of the refrigerant vapour under given operating conditions. It is found that for a certain mass flow rate of refrigerant, sub–cooling does not occur at some higher ambient temperatures, thus showing the inefficiencies of the heat exchanger. The methodology can be used as a design tool for the design of wire–and–tube condenser of a small refrigerator. The derating of condenser under abnormal ambient conditions can also be predicted. Keywords: Wire-and-tube condenser, refrigerator, free convection, finite element method, phase change

Experimental investigation of film boiling on spheres using high-speed video

The experimental investigation of saturated Freon-113 film boiling on spheres with different diameters at atmospheric pressure under conditions of free convection is executed. It was found that with increasing diameter of the sphere and the temperature difference is changing the wave motion of the vapor film with two-dimensional to three-dimensional mode. Also, found that in a range of regime parameters at which observed a three-dimensional interface motion, the destruction method of two-dimensional wave is similar to a series of three or more waves. I.e. was some system memory. When the temperature difference close to critical after the passage of a wave are possible the local contacts of liquid with a heated surface of the sphere. However, these contacts do not lead to degradation of the wave motion of the interface, and the film boiling crisis of saturated Freon-113 occurs smoothly in contrast to the crisis at boiling of saturated and subcooled water.