Natural Convection Conditions Research Articles

CFD evaluations on bundle effects for steam condensation in the presence of air under natural convection conditions

Steam condensation in the presence of air is a relevant phenomenon in various industrial applications. Its heat transfer property has been broadly revealed by experimental and numerical investigations on vertical plates or single tubes. However, these results may not be directly applied to tube bundle cases, since there may exist mutual interactions among adjacent tubes. To have an insight on this problem, the present work conducted numerical simulations on various tube bundles with a tube pitch 1.5 times of the tube diameter. The cases evaluated were classified into three categories including the single row, double row, and triple row. In each category, structures with various tube columns were assessed. The results indicate that tube bundles will thicken the near-wall high concentration air layer, resulting in inhibited condensation heat transfer. This phenomenon is defined as the inhibition effect. On the other hand, a heat transfer enhancement effect caused by a strengthened natural circulation driven by the density difference between the mainstream and the high concentration air region is found and defined as the stack effect. The average condensation heat transfer for tube bundles is determined by the relative magnitude between the inhibition effect and the stack effect. The stack effect becomes much intensive with the increase of tube rows and columns, which can enhance condensation heat transfer. Typically, the results in the triple row categories show that the maximum average heat transfer coefficient can be 14% greater than that of the single tube.

To the Issue of Choosing a Wastewater-Freon Heat Exchanger for a Vapor Compression Pump

A high level of energy-efficient operation of structures can be achieved through automated life-support systems using secondary energy resources. Analysis of thermal emissions in residential buildings allows us to conclude that the most promising for this purpose are wastewater. The process of utilizing the heat of the used water resources entering the sewerage network can be carried out in two types of evaporative heat exchangers: surface and submerged.The increased area and increased heat transfer efficiency create the conditions for the preferable choice of a submersible type storage wastewater-freon heat exchanger. The design of the case of devices of this type allows to obtain a model range by the step change in its diameter. The calculations carried out for the case diameters of 300, 400, 500 mm have shown that at various water flows the best heat exchange parameters are achieved at a diameter of 300 mm, both under conditions of forced and natural convection.

Heat transfer performance testing of a new type of phase change heat sink for high power light emitting diode

In view of the limitations of solid metal heat sink in the heat dissipation of high power light emitting diode (LED), a kind of miniaturized phase change heat sink is developed for high power LED packaging. First, the fabrication process of miniaturized phase change heat sink is investigated, upon which all parts of the heat sink are fabricated including main-body and end-cover of the heat sink, the formation of three-dimensional boiling structures at the evaporation end, the sintering of the wick, and the encapsulation of high power LED phase change heat sink. Subsequently, with the assistance of the developed testing system, heat transfer performance of the heat sink is tested under the condition of natural convection, upon which the influence of thermal load and working medium on the heat transfer performance is investigated. Finally, the heat transfer performance of the developed miniaturized phase change heat sink is compared with that of metal solid heat sink. Results show that the developed miniaturized phase change heat sink presents much better heat transfer performance over traditional metal solid heat sink, and is suitable for the packaging of high power LED.

Experimental study of heat transfer on the rubber-cord cased pneumatic element surfaces under the natural convection

One of the ways of improving the calculations accuracy of the stationary objects and vehicles protection systems from vibrations and impacts is taking into account the influence of heat exchange processes with the environment occurring in the rubber-cord cased pneumatic elements and obeying Fourier’s law of heat conduction and Newton’s heat-transfer law. In the conducted study, the heat transfer coefficient values on the pneumatic element walls in the natural convection conditions are defined experimentally. The test bench, testing technique and used measuring instruments which are the heat flux sensors with the internal thermocouple are described. The heat transfer coefficient on the rubber-cord casing curved surface and on the metal flanges flat ones is found out to assume constant values being sufficient for practical application in the operating temperature range based on the process data. When applying the heat exchangers technical theory methods for the pneumatic elements calculation, it is acceptable to consider as a first approximation that the heat transfer coefficient has the identical values on all outer surfaces of the pneumatic element.

Convective ignition of polymers: New apparatus and application to a thermoplastic polymer

A new convective ignition apparatus for polymers has been developed with measured flow and temperature fields. Polymer degradation and ignition is typically studied in fire science under radiative heating or direct contact with a pilot flame but this new apparatus allows for research to be conducted in a convective setting providing a missing piece of knowledge on flammability. Convective heating is a main mode of heat transfer in many real fires such as in the built or natural environment, like building fires or wildfires. The apparatus exposes one side of a sample to air between lab ambient and 735 °C at 0.7 to 5 m/s whilst measuring the sample 2D back side temperature via calibrated infrared. The 2D temperature and flow fields, convective heat transfer, and irradiation were studied under various operating conditions of temperature and flow. Polybutylene terephthalate (PBT) samples with glass fibre were ignited using a 735 °C hot stream. Samples of 2 mm thickness ignited after 30 s with a standard deviation lower than 1 s. The experimental work was augmented with numerical modelling of heat and mass transfer with pyrolysis chemistry in Gpyro, allowing for insight into the temperatures across the sample. Combining experimental with numerical work shows that ignition was observed at a surface temperature of 320 °C. Using this rig, ignition can be studied under a range of temperature and flow conditions filling the gaps of the literature which relies primarily on irradiated samples in natural convection conditions.

The effect of airspeed and wind direction on human's thermal conditions and air distribution around the body

The purpose of this research was to investigate the effects of airspeed and wind direction on a human's thermal conditions and of air distribution around the body. In addition, this study presents the equivalent temperature, which is the most effective index to evaluate the effect of air movement. The effect of convection and radiation on each part of the human body was estimated through numerical calculation, and the results were verified through a combination of thermal manikin experiments and radiation analysis. The experiments were conducted in a climate chamber and wind tunnel under conditions of natural convection and forced convection, respectively. An airspeed of 0.05 m/s was selected for the natural convection condition and an airspeed range of 0.2–2.0 m/s for forced convection conditions. The natural convective heat transfer coefficient in the stagnant environment was 3.27 W m−2 K−1 in the sitting posture and 2.67 W m−2 K−1 in the standing posture because the natural upward airflow caused by human body heat is more likely to diffuse to the surrounding environment in a sitting posture than in a standing posture. The equivalent temperature change of the whole body was greater in the standing posture than in the sitting posture when the airspeed was 0–0.2 m/s. The equivalent temperature change of the whole body was similar in both sitting and standing postures when the airspeed was greater than 0.2 m/s.

Design and Optimization of Thermoelectric Cooling System Under Natural Convection Condition

A design method for the thermoelectric cooling system is improved in this work based on a graphical approach. It is used to select an appropriate thermoelectric cooler (TEC) and determine the value of optimum input current. Theoretical analysis has been conducted to investigate the cooling performance of the system using the design method. Numerical simulation and experimental tests for the entire cooling system validate the calculation result, which indicates the high reliability of the theoretical design method. The temperature dependence of the heat sink resistance and the contact resistance are the major reasons for the small discrepancy. Research is then conducted based on the design method to investigate how a thermoelectric cooling system under natural convection performs, where the optimization of heat sinks at hot side of TEC is done by using the generalized correlations in the previous studies. Comparison is made between the thermoelectric cooling system and the bare-heat-sink system under natural convection. Results show that the thermal resistance of the heat sink attached to TEC is critical to the cooling performance of the whole system. Besides, TEC under natural convection can perform better than the passive cooling if the heat load is not very high (qc″≤20,000 W/m2). The design process and results can provide a useful guidance for other thermal engineers.

Natural convection heat transfer of a straight-fin heat sink

The influence of mounting angle on heat dissipation performance of a heat sink under natural convection condition is investigated in this paper by numerical simulation and experimental test. It is found that the heat sink achieves the highest cooling power when its mounting angle is 90°, while it reaches the lowest when the mounting angle is 15°, which is 6.88% lower than that of 90°. A heat transfer stagnation zone is the main factor that affects the cooling power of the heat sink, and its location and area vary with the mounting angle. It is identified that cutting the heat transfer stagnation zone is an effective way to improve the heat sink performance.

The role of surface wettability on natural convection frosting: Frost growth data and a new correlation for hydrophilic and hydrophobic surfaces

In refrigeration systems, frost accumulation on the evaporator demands periodic defrosting. Chemical coatings (either hydrophilic or hydrophobic) are sometimes used to mitigate frost build-up and/or improve drainage during the defrost cycle. Although several studies have been performed on frost growth behavior, relatively few studies have specifically examined the effect of surface wettability on the frost growth rate. This paper focuses on the influence of environmental operating conditions on the growth of a frost layer on surfaces which span a wide range of contact angles under natural convection conditions. A new semi-empirical correlation which includes the surface contact angle θ and the modified Jakob number Λ as parameters is reported with an average predictive error of 11.7% (N = 930) for all non-zero experimental data. The correlation was developed using multivariable regression analysis and data which span different surface temperatures (−13 °C to −5°C), relative humidities (40–80%), and static contact angles ranging from 45° (hydrophilic) to 160° (hydrophobic).

Convection Heat Transfer Studies on Rectangular Fin Arrays with Different Surface Roughness, Perforations or Protrusions on Fins – a Review

The present paper summarizes the review of numerical and experimental works done by researchers to study various types of fin configurations under natural convection conditions to increase heat transfer with different fin surface roughness. Compared to plane fin, the fin with varying surface roughness influence heat transfer in varying degrees. Fins with perforations of rectangular, circular, triangular shape etc. and fins with extensions or protrusions of various shapes have been studied. Such perforations / protrusions induce flow discontinuity over fins and hence help to alter flow quantities near the wall. Thus generated instability in boundary layer increase heat transfer from fins. In some studies, it was found that roughness height plays an important role and is a decisive factor in increasing or decreasing heat transfer from such surfaces. In other studies, roughness patterns considerably enhance heat transfer from 10 to 40 % compared to plane fin. As a future scope, fins with various knurling patterns and slit openings of various sizes and shapes seem promising in assessing heat transfer increase.

Prognostic potential of free convection models for analysis of thermal conditions of heat supply objects

The article shows the results of mathematical simulation of convective turbulent heat transfer in the closed domain with heat-conducting walls and the source of heat emission. The system of equations in the model of thermal conductivity for the solid walls and Navier-Stokes equations for gas are solved. The article examines the possible versions of the calculation of the turbulent regime in the geometrically simple air region by means of conducting the simulation within the framework of algebraic models (Van Driest and Prandtl-Reichardt), and k-? model. On the basis of the obtained results the authors made a conclusion about the possibility of applying the algebraic model of Prandtl to describe the integral characteristics of turbulent flows in the conditions of natural convection in a geometrically simple area when the air heated by the heat source is moved by the lifting force. Besides, the temperature fields for a typical real object of heat supply are simulated in the article. The values of the dimensionless heat exchange coefficient at the air-wall interface are determined. The comparative analysis of two quite significantly different approaches to determine the average temperature in the heated room, i. e. the traditional balance approach and the approach based on the considered system of partial differential equations is executed. It is concluded that the balance models of the calculation of the temperature regime can adequately describe real temperatures of heat supply objects only for very large values of the characteristic times of the processes in question.

Experimental study on thermal characteristics and temperature distribution of laminated lithium-ion power battery

Thermal issues are increasingly critical for the scaling-up and integrated deployment of laminated lithium-ion batteries (LIB). Numbers of studies have been conducted on the thermal management for battery packs, however, the temperature behavior for battery cells are not get enough attention, which is the basis for battery thermal management. Thus, infrared imaging technology is proposed and applied in our study to investigate the temperature distribution for a Li(NiCoMn)O2 battery in the condition of natural convection. The temperature variance (Tvar2) is proposed and calculated to assess the temperature distribution for battery cells. Results show that (1) the heat generation rate of the positive electrode is higher than that of the negative electrode; (2) when the current collector is arranged in the same direction, the heat generation rate close to the current collector is higher than the areas without current collators; (3) The battery cell performs better in terms of temperature distribution when the current collators are arranged in the opposite direction than that in the same direction (4) With the increase of discharge rate and discharge depth, the temperature distribution shows increasingly non-uniformity trend.

Potassium-cobalt sulphate crystal growth assisted by low frequency vibrations

Single crystals of K2Co(SO4)2·6H2O were grown from solution using the temperature reduction method enhanced by the axial low frequency vibration control technique (AVC-technique). Physical modeling of heat-mass transfer in solution under the AVC action was performed. The growth rate of the AVC grown crystal was found to be twice that of the crystal grown under natural convection conditions. Analysis of spectral characteristics (absorption and Raman spectra) as well as structural properties (dislocation density and microhardness) of the grown crystals showed the significant superiority of the AVC technique for the growth of K2Co(SO4)2·6H2O crystals.

Enhancement of Condensation Heat Transfer Rate of the Air-Steam Mixture on a Passive Condenser System Using Annular Fins

This paper presents an experimental investigation on the enhancement of the heat transfer rate of steam condensation on the external surfaces of a vertical tube with annular fins. A cylindrical condenser tube, which is 40 mm in outer diameter and 1000 mm in length, with annular disks of uniform cross-sectional area is fabricated in the manner of ensuring perfect contact between the base surface and fins. A total of 13 annular fins of 80 mm diameter were installed along the tube height in order to increase the effective heat transfer area by 85%. Through a series of condensation tests for the air-steam mixture under natural convection conditions, the heat transfer data was measured in the pressure range of between 2 and 5 bar, and the air mass fraction from 0.3 to 0.7. The rates of heat transfer of the finned tube are compared to those that are measured on a bare tube to demonstrate the enhanced performance by extended surfaces. In addition, based on the experimental results and the characteristics of steam condensation, the applicability of finned tubes to a large condenser system with a bundle layout is evaluated.

Effect of the ambient temperature and oxygen concentration in droplet combustion under natural convection conditions

A numerical investigation of combustion of n-heptane droplets in natural convection conditions is presented. The focus is on the effects of ambient temperature and oxygen concentration on the combustion characteristics. Ambient temperatures range from 900K to 1400K, and volume fraction of oxygen range from 17% to 21%, and pressure is 0.1MPa. The results show that ambient temperature and oxygen concentration have a great influence on burning rates and ignition delay times. The ignition delay times decreases with increasing temperature. In addition, the burning rates of with and without natural convection compare to existing theoretical model. The burning rate increase under natural convection.

Experimental investigation of the daily thermal performance of a mPCM honeycomb wallboard

This study combined building construction practice, aluminum honeycomb panels, and microencapsulated phase change material (mPCM) to construct an mPCM honeycomb wallboard. The daily thermal behaviors of this wallboard are investigated experimentally. The outdoor side of the wallboard is affected by daytime sunshine and nighttime natural convection, whereas the indoor side is subjected to forced convection or natural convective boundaries. The results show that indoor air conditions have a certain influence on the thermal performance of mPCM honeycomb wallboard and that the mPCM inside the wallboard can effectively dissipate heat when the indoor environment is subjected to either a forced or natural convection condition. Therefore, mPCM can always restore itself to its original thermal condition and is ready for heat management use the following day.

A semi-empirical model for predicting frost accretion on hydrophilic and hydrophobic surfaces

A first-principles model was used together with experimental data obtained in-house to validate a semi-empirical modeling approach for predicting frost accretion on hydrophilic and hydrophobic substrates. An algebraic expression for the frost thickness as a function of the time, the modified Jakob number, the humidity gradient, and the surface contact angle was devised from frost formation theory. At this stage, the correlation was fitted to 956 experimental data points for natural convection conditions spanning different surface temperatures and supercooling degrees, with the modified Jakob number ranging from 0.79 to 1.30, and contact angles ranging from 45° (hydrophilic) to 160° (hydrophobic). When compared to the experimental data for the frost thickness, the proposed semi-empirical model showed errors within ±15% bounds, and an average predictive error of 11.7%. Since the model carries the contact angle as an independent parameter, a sensitivity analysis of the frost growth rate in relation to it is also reported.

Study of underfloor air distribution using zonal model-based simulation and experimental measurements

Underfloor air distribution (UFAD) is an effective strategy to provide both ventilation and air conditioning for buildings. To predict indoor airflows and temperature distribution, zonal model has advantages over other methods such as multi-zone model and computational fluid dynamics (CFD) by considering the prediction accuracy and computational cost. In this work, a numerical algorithm for floor-mounted diffuser flows, which calculates the airflow mass conservation and energy conservation separately, was implemented in a newly developed zonal model computer program for UFAD application, based on the established zonal model-pressurized zonal model with air-diffuser (POMA). This computer program was validated by lab experiments under both natural convection condition and mechanical ventilation for a lab-scale mixing UFAD system. Comparisons of zonal model simulation results and experimental measurements for both temperature distributions and air flows in the lab room showed that the newly developed zonal model is a good tool for predicting the indoor thermal environment with UFAD system.

Temperature and velocity measurements in a buoyant flow induced by a heat source array on a vertical plate

Heat source arrays are common in engineering applications. Natural convection is a reliable and silent cooling strategy. Therefore, an array of flush-mounted heat sources has been studied under conjugate conduction and natural convection condition. This studies was performed for a system with relatively large dimensions, typical for power electronics, and a modified Rayleigh number up to 2·1010. A modular set of heaters was designed to vary the distribution of heat sources on the plate and investigate the influence of the spacing. Velocity and temperature were measured in the convective flow with particle image velocimetry and micro-thermocouple. The velocity field was analyzed with proper orthogonal decomposition. The first instabilities of the convective flows were described. The results gave a better understanding of the heat transfers in these configurations and are valuable for model validation.

Numerical investigation on the steady temperature rise of a by-pass switch

The quench protection switch (QPS) is very important for ensuring the safety of the magnet coils of a superconducting Tokamak. The main function of a QPS is to protect the magnet as the coil quench occurs. Besides, a QPS has to withstand almost all of the coil current of some tens of kA flowing through it in the normal operation condition. This task is undertaken by the by-pass switch (BPS), which is an indispensable component of a QPS, no matter what principle the QPS is based on. Therefore, the study of the temperature rise is very important for optimal design of a BPS. In this paper, a scheme of a high-current BPS is proposed. The steady temperature rise of the BPS is simulated in the condition of natural convection with the approach of Computational Fluid Dynamics (CFD). Furthermore, the influence of the space between each individual switch unit is investigated. Simulation results indicate that there exists an appropriate space, beyond which the mutual influence of adjacent contacts can be accepted. This space can be regarded as a reference in designing a BPS with the trade-off between the dimension of the BPS (and the QPS) and its current-carrying capacity.