Natural Convection Mode Research Articles

Heat Transfer Study in Oil Channels of a Power Transformer Winding ONAN Cooling System with and without Insulating Paper Based on Numerical Modeling

In this study 3 geometry for ONAN cooling system of power transformers are numerically simulated with (Perforated nomex) and without insulating papers (Netting tape). Geometries are A, RA, and R type. The A type has just axial channels. The RA type has radial and axial channels but there are no baffles to drive the flow into radial channels. The R type is like the RA type but with baffles. Insulating paper used in this study has some holes to cause some vortex in flow to increase heat transfer. The CFD code used is Fluent 6.3. To simulate flow in natural convection mode, Boussinesq model and pressure inlet and outlet boundary conditions are used. Results show that the most effective geometry is the one with baffles that drive flow in radial channels in addition to axial channels. Streamlines near the holes in papers show that that height of holes is too small to affect heat transfer and, so, the comparison between geometries with and without paper shows the types without paper have higher heat transfer coefficients.

ANFIS modelling of a natural convection greenhouse drying system for jaggery: an experimental validation

The aim of the current research work is to generate an adaptive-network-based fuzzy inference system (ANFIS) model so as to forecast the jaggery temperature, the greenhouse air temperature and the moisture evaporation for drying of jaggery inside the greenhouse for natural convection mode. The experiment was conducted separately for 0.75 and 2.0 kg of jaggery pieces having dimensions of 0.03×0.03×0.01 m3 for complete drying. The jaggery was dried in a roof-type even span greenhouse with a floor area of 1.20×0.78 m2. An ANFIS model was developed in MATLAB software so as to calculate the jaggery temperature, the greenhouse air temperature and the moisture evaporated and was also used to forecast the thermal performance of the greenhouse on the basis of solar intensity and ambient temperature. This model was experimentally validated. It was shown that the analytical and experimental results for jaggery drying are in good agreement.

Computational analysis of thermo-hydraulic behavior of TRIGA research reactor

Key thermal hydraulic parameters of the 3MW TRIGA Mark-II research reactor operating under steady-state conditions were investigated using the thermal hydraulic codes NCTRIGA, PARET and COOLOD-N2. Results of the neutronic analysis performed by 3-D Monte Carlo code MCNP4C were used in NCTRIGA and coupled output of neutronic analysis carried out by using 3-D diffusion code CITATION and 3-D Monte Carlo code MCNP4B2 were used in the PARET to study the steady-state thermal hydraulic behavior of the reactor. To benchmark the NCTRIGA, PARET and COOLOD-N2 models, data were obtained from different measurements executed by thermocouples in the instrumented fuel elements (C1 and D2) and the hottest fuel element (C4) during the steady-state operation both under forced and natural convection mode and compared with the calculation found to be quite consistent. The mass flow rates needed for input to PARET and COOLOD-N2 were taken from final safety analysis report (FSAR) for a downward forced coolant flow equivalent to 3500gpm. For natural convection cooling of reactor, mass flow rate was generated using NCTRIGA code. The testing of the NCTRIGA, PARET and COOLOD-N2 model calculations through benchmarking the available TRIGA experimental and operational data showed that NCTRIGA, PARET and COOLOD-N2 codes can successfully be used to analyze the thermal hydraulic behavior of the reactor for the steady-state operation under both natural and forced convection mode of coolant flow to predict the safety margins. The result obtained in this investigation can be used for upgrading the current core configuration of the TRIGA reactor.

Study on heat transmission through multilayer clothing assemblies under different convective modes

The present work reports a detailed study on dry thermal resistance of multilayered clothing assemblies under different modes of heat transmission (i.e. non-convective mode, natural convection and forced convection). A series of multilayered fabric assemblies have been created with different combinations of fabric layers (like plain woven, nonwoven wadding, warp-knitted spacer fabric, weft-knitted breathable coated and weft-knitted aluminium coated) and air gaps of different thicknesses between fabric layers. Significant impacts of thickness of air gaps and mode of heat transmission on thermal resistance of multilayered clothing assemblies have been observed. The thermal resistance of the fabric assemblies increases with the increase in the thickness of air gap in between fabric layers. The thermal resistance of all the fabric assemblies at any given air layer thickness is highest under non-convective mode of heat transmission, followed by natural convection mode and the forced convection mode shows least thermal resistance. Incorporation of coated fabric in the outer layer reduces the effect of forced convection. ANOVA has been prepared and F value has been calculated to find the effect of modes of convection, type of fabric and thickness of air layer on thermal transmission properties. The results obtained from this detailed study will help in designing multilayered clothing assemblies suitable for different climatic conditions.

Characterization of working fluid in vertically mounted finned U-shape twin heat pipe for electronic cooling

As part of the ongoing research on finned U-shape heat pipes for CPU cooling, the present work focuses on the characterization of working fluid in vertically oriented twin U-shape heat pipe, by taking into account the gravity of flow. Two-dimensional FE simulation is performed under natural and forced convection modes, by using ansys-flotran. The best heat input and coolant velocity for the simulations are determined experimentally, corresponding to the least thermal resistance. The wall temperatures at the evaporator, adiabatic and condenser sections, and the velocity and pressure distributions of vapor and liquid, are analyzed. The total heat input for minimum thermal resistance in both natural and forced convection is found to be 50W, and the coolant velocity is 3m/s. The predicted and experimental wall temperatures are found in excellent match. It is observed that for the present U-shape heat pipe configuration, the difference in evaporator and condenser temperatures is significantly small, resulting in enhanced heat transfer compared to the conventional heat pipes. The sintered copper wick has a small pore size, resulting in low wick permeability, leading to the generation of high capillary forces for anti-gravity applications.

MODELLING AND SIMULATION OF A MODULAR SOLAR STILL CONSTITUTED OF A STREAMING PLATE COLLECTOR AND A CONDENSATION CHAMBER

In this paper, we present a numerical study of a modular solar distillation system working in natural convection mode. This still is constituted of a streaming plan solar collector and a condensing chamber. Equations governing the heat and mass exchange are established using the nodal method. The operation of the still is simulated for a typical day of the month with the weather conditions of Algiers (Algeria). Results show that solar radiation is the most important parameter of the system operation. The obtained distillate mass is maximum during the typical day of August, while the best performance of the condensing chamber is recorded during May. DOI: http://dx.doi.org/10.5755/j01.mech.18.1.1281

Construction and Performance Analysis of an Indirect Solar Dryer Integrated with Solar Air Heater

The use of conventional sources of energy for drying agricultural product is an expensive process in developing countries. Therefore, an appropriate technology for drying of agricultural produce has been developed and its performance for the drying of tomatoes was evaluated. An indirect type natural convection solar dryer was developed. The aim of this study is to evaluate dryer performance under natural and forced convection modes of drying. In natural and forced convection, air mass flow rates of 0.00653kg/s and 0.019kg/s respectively were obtained. Hot air reached an average drying temperature of 45°C in natural convection and 40°C in forced convection. The results obtained from natural and forced convection modes of drying were also compared with open sun drying. The colour, flavour and time required for drying is favorable in solar drying as compared to sun drying.

Effect of Convection Mode on Radiation Heat Transfer Distribution in Domestic Baking Oven

The aim of this study is to analyze on the effect of radiation heat transfer inside an oven under natural and forced convection modes using network representative method. This analysis is important to understand the fundamental principle of heat transfer occur in an oven. Most previous work focused on conduction and convection process in an oven instead of radiation which leads to this research. The experiment was conducted using the baking oven with temperature ranges from 180° C, 200° C and 220° C. The oven was pre-heated 10 minutes prior taking the temperature reading for duration of maximum 20 minutes. The data collected were recorded in the data logger. Based on these data, analysis on the radiation exchanges that occurred inside the oven chamber were performed by the network representative method. The radiation rates of all surface involved were successfully determined. With the calculated radiation rate, analysis on the effects of radiation under natural convection and forced convection modes were performed. Based on the temperature profiles and radiation rate patterns, it was proven that the forced convection mode has more radiation effect compared to natural convection mode. chamber. The radiation analysis of heat transfer starting from the oven heating element to the surrounding chamber was analyzed in details by using the network representative method. This method based on the electrical network that shows the surface radiation exchange inside the oven chamber. Based from the radiation analysis under natural convection and forced convection process, this could help to improve a better implementation of heat transfer studies in designing an oven.

Experimental analysis and FEM simulation of finned U-shape multi heat pipe for desktop PC cooling

This paper presents the performance analysis of a finned U-shape heat pipe used for desktop PC-CPU cooling. The experiments are conducted by mounting the system vertically over a heat source situated inside a rectangular tunnel, and force convection is facilitated by means of a blower. The total thermal resistance ( R t ) and heat transfer coefficient are estimated for both natural and forced convection modes under steady state condition, by varying the heat input from 4 W to 24 W, and the air velocity from 1 m/s to 4 m/s. The coolant velocity and heat input to achieve minimum R t are found out and the corresponding effective thermal conductivity is calculated. The transient temperature distribution in the finned heat pipe is also observed. The experimental observations are verified by simulation using ANSYS 10. The results show that the air velocity, power input and heat pipe orientation have significant effects on the performance of finned heat pipes. As the heat input and air velocity increase, total thermal resistance decreases. The lowest value of the total thermal resistance obtained is 0.181 °C/W when heat input is 24 W and air velocity 3 m/s. The experimental and simulation results are found in good agreement.

A simplified model for understanding natural convection driven biomass cooking stoves—Part 1: Setup and baseline validation

It is estimated that half the world's population cooks over an open biomass fire; improved biomass cooking stove programs have the potential to impact indoor air quality, deforestation, climate change, and quality of life on a global scale. The majority of these cooking stoves operate in a natural convection mode (being driven by chimney effect buoyant fluid forces). Simplified theories for understanding the behavior of this unexpectedly complex combustion system, along with practical engineering tools to inform its design, are markedly lacking. A simplified model of the fundamental stove physics is developed for predicting bulk flow rate, temperature, and excess air ratio based on stove geometry (chimney height, chimney area, viscous and heat release losses) and the firepower (as established by the stove operator). These parameters are intended to be fundamental inputs for future work understanding and improving biomass cook stove emissions and heat transfer. Experimental validation is performed and the simplified model is shown to be accurate and applicable to typical stove operation. Carbon monoxide and particulate matter emissions data have been recorded in conjunction with the validation data. The initial results are presented and indicate that the excess air ratio is a promising tool for reducing carbon monoxide emissions.

CFD-Assisted Optimization of Chimneylike Flows to Cool an Electronic Device

Natural convection cooling provides a reliable, cost-effective, energy-efficient and noise-free method to cool electronic equipment. However, the heat transfer coefficient associated with natural convection mode is usually insufficient for electronic cooling and it requires enhancement. Chimneylike flows developed within the cabinets of electronic devices can provide better mass flow and heat transfer rates and can lead to greater cooling efficiency. Constraints in the design of natural convection cooling systems include efficiency of packing, aesthetics, and concerns of material reduction. In this paper, methods based on computational fluid dynamics are used to study the effects of parameters such as (1) vertical alignment of the slots, (2) horizontal alignment of slots, (3) area of slots, (4) differential slot opening, and (5) zonal variation in heat generation on natural convection cooling within such design constraints. Insights thus derived are found useful for designing an energy-efficient and ecofriendly cooling system for electronic devices.

Unsteady conjugate natural convection in a square enclosure filled with a porous medium

Mathematical simulation of unsteady natural convection modes in a square cavity filled with a porous medium having finite thickness heat-conducting walls with local heat source in conditions of heterogeneous heat exchange with an environment at one of the external boundaries has been carried out. Numerical analysis was based on Darcy–Forchheimer model in dimensionless variables such as a stream function, a vorticity vector and a temperature. The special attention was given to analysis of Rayleigh number effect Ra = 10 4, 10 5, 10 6, of Darcy number effect Da = 10 −5, 10 −4, 10 −3, ∞, of the transient factor effect 0 < τ < 1000 and of the heat conductivity ratio k 2,1 = 3.7 × 10 −2, 5.7 × 10 −4, 6.8 × 10 −5 on the velocity and temperature fields. The influence scales of the defining parameters on the average Nusselt number have been detected.

Numerical investigation of heat transfer in Plastic Leaded Chip Carrier (PLCC) packages in in-line arrangement

Plastic Leaded Chip Carrier (PLCC) package has been emerged a promising option to tackle the thermal management issue of micro-electronic devices. In the present study, three dimensional numerical analysis of heat and fluid flow through PLCC packages oriented in-line and mounted horizontally on a printed circuit board, is carried out using a commercial CFD code, FLUENTTM. The simulation is performed for three configurations such as 4PLCC, 8PLCC and 12PLCC under natural, mixed and forced convection modes with different inlet velocities and chip powers. The contours of average junction temperatures are obtained for each package under different conditions. It is observed that the junction temperature of the packages decreases with increase in inlet velocity and increases with chip power. Moreover, the increase in package density significantly contributed to rise in temperature of chips. Thus the present simulation demonstrates that the chip density (the number of packages mounted on a given area), chip power and the coolant inlet velocity are strongly interconnected; hence their appropriate choice would be crucial. Keywords: PLCC package, Thermal management, Numerical simulation, Average junction temperature

Simulation of mixed convection in slender rectangular cavity with lattice Boltzmann method

PurposeThe purpose of this paper is to apply the lattice Boltzmann method (LBM) to simulate mixed flow, which combines natural convection for temperature difference and forced convection for lid driven, in a two‐dimensional rectangular cavity over a wide range of aspect ratios (A), Rayleigh numbers (Ra) and Reynolds numbers (Re).Design/methodology/approachThe LBM is applied to simulate the mixed flow. A multi‐relaxation technique was used successfully. A scale order analysis helped the understanding and predicting the overall heat transfer.FindingsIn the considered lid driven cavity, the Richardson number emerges as a measure of relative importance of natural and forced convection modes on the heat transfer. An expression of the overall heat transfer depending on the cavity slender (A) is deduced. The validity of the obtained expression was checked in mixed convection under the condition of low Richardson number (Ri) and the limitation condition was deduced.Practical implicationsThis paper has implications for cooling system optimization and LBM technique development.Originality/valueThis paper presents a new cooling configuration, avoiding critical situation where the opposing effect induce weak heat transfer; and a stable and fast LBM approach allowing complex geometry treatment.

Transient Mixed Convection in a Two-Sided Lid-Driven Differentially Heated Square Cavity Utilizing Nanofluids

Transient behaviour of mixed convection inside a two-sided lid-driven differentially heated square cavity filled with nanofluids is investigated numerically to gain insight into convective recirculation and flow processes evolved with time. A model is developed to analyse the behaviour of nanofluids taking into account the solid volume fraction (π). The transport equations are solved numerically using the finite volume approach and a SIMPLE algorithm on a non-staggered grid with deferred QUICK scheme. Comparisons with previously published work are performed and found to be in excellent agreement. The solution procedure is moved with small values of τ so that a transient behaviour can be captured. Richardson number, Ri = Gr/ Re2, a measure of relative importance of natural and forced convection modes on the heat transfer, is taken as 1 so that the phenomena of mixed convection can be understood effectively. Copper—water nanofluid is used with Pr = 6.2 and solid volume fraction π is varied as 0.0, 0.08, and 0.16. Detailed results are presented for streamline and isotherm patterns. A comparison of average Nusselt number shows that heat transfer is more in the case of π = 0.16. The present study focuses on the transient analysis of several parameters on the heat transfer characteristics of nanofluids within the enclosure.

Improvement in greenhouse solar drying using inclined north wall reflection

A conventional greenhouse solar dryer of 6 m 2 × 4 m 2 floor area (east–west orientation) was improved for faster drying using inclined north wall reflection (INWR) under natural as well as forced convection mode. To increase the solar radiation availability onto the product (to be dried) during extreme summer months, a temporary inclined wall covered with aluminized reflector sheet (of 50 μm thickness and reflectance 0.93) was raised inside the greenhouse just in front of the vertical transparent north wall. By doing so, product fully received the reflected beam radiation (which otherwise leaves through the north wall) in addition to the direct total solar radiation available on the horizontal surface during different hours of drying. The increment in total solar radiation input enhanced the drying rate of the product by increasing the inside air and crop temperature of the dryer. Inclination angle of the reflective north wall with vertical ( β) was optimized for various selective widths of the tray W (1.5, 2, 2.5 and 3 m) and for different realistic heights of existing vertical north wall ( h) at 25°N, 30°N and 35°N latitudes (hot climatic zones). Experimental performance of the improved dryer was tested during the month of May 2008 at Ludhiana (30.56°N) climatic conditions, India by drying bitter gourd ( Momordica charantia Linn) slices. Results showed that by using INWR under natural convection mode of drying, greenhouse air and crop temperature increased by 1–6.7 °C and 1–4 °C, respectively, during different drying hours as compared to, when INWR was not used and saved 13.13% of the total drying time. By using INWR under forced convection mode of drying, greenhouse air and crop temperature increased by 1–4.5 °C and 1–3 °C, respectively, during different drying hours as compared to, when INWR was not used and saved 16.67% of the total drying time.

Mathematical modelling and experimental investigation on sun and solar drying of white mulberry

The drying kinetics of white mulberry was investigated in a solar dryer with forced convection and under open sun with natural convection. The constant rate period is absent from the drying curve. The drying process took place in the falling rate period. The drying data were fitted to the different mathematical models. The performance of these models was investigated by comparing the determination of coefficient (R), reduced chi-square (χ2) and root mean square error (RMSE) between the observed and predicted moisture ratios. Among these models, the drying model developed by Logarithmic model showed good agreement with the data obtained from the experiments in the solar dryer with forced convection drying mode. The Verma et al. model has shown a better fit to the experimental mulberries data for open sun drying with natural convection mode than the other models.

Dynamic Behaviour of a Continuous Heat Exchanger/Reactor after Flow Failure

The intensified technologies offer new prospects for the development of hazardous chemical syntheses in safer conditions: the idea is to reduce the reaction volume by increasing the thermal performances and preferring the continuous mode to the batch one. In particular, the Open Plate Reactor (OPR) type ``reactor/exchanger" also including a modular block structure, matches these characteristics perfectly. The aim of this paper is to study the OPR behaviour during a normal operation, that is to say, after a stoppage of the circulation of the cooling fluid. So, an experiment was carried out, taking the oxidation of sodium thiosulfate with hydrogen peroxide as an example. The results obtained, in particular with regard to the evolution of the temperature profiles of the reaction medium as a function of time along the apparatus, are compared with those predicted by a dynamic simulator of the OPR. So, the average heat transfer coefficient regarding the ``utility" fluid is evaluated in conductive and natural convection modes, and then integrated in the simulator. The conclusion of this study is that, during a cooling failure, a heat transfer by natural convection would be added to the conduction, which contributes to the intrinsically safer character of the apparatus.

Evaluation of convective heat and mass transfer in Open Sun and Green House Drying

In this paper, a study on open sun and green house drying has been performed to evaluate the convective heat and mass transfer coefficients as a function of climatic parameters. The hourly data for the rate of moisture removal, crop temperature, relative humidity inside greenhouse and outside greenhouse and ambient air temperature for complete drying have been recorded. The experiments were done in the premises of Allahabad Agricultural Institute-Deemed University, after the crop-harvesting season from February 2006 to April 2006. Using these data, the value of convective heat and mass transfer in Open Sun Drying (OSD) and Green House Drying (GHD) under natural and forced convection mode have been calculated. On the basis of this study, it is inferred that the value of convective heat and mass transfer is more in OSD than in the Green House Drying (GHD) under natural convection mode. However, its value increases in forced mode of GHD when compared with natural mode in the initial stage of drying.

Investigation of thermohydraulic parameters during natural convection cooling of TRIGA reactor

Important steady-state thermohydraulic parameters of the TRIGA research reactor operating under natural convection mode of coolant flow were investigated using NCTRIGA computer code. Neutronic parameters used in preparing the input of NCTRIGA were taken from the analysis performed by 3-D Monte Carlo code MCNP4C. Benchmarking of the NCTRIGA calculated results were performed against the experimental data measured by the thermocouples in the instrumented fuel element (IFE) during the steady state operation of the reactor under natural convection mode of coolant flow. Various thermohydraulic parameters like the coolant velocity, flow rate and mass flow rate were generated for the hot channel as well as for the two channels comprising instrumented fuels. Calculated peak fuel temperatures at different power levels were compared with the measured values and also with the calculations performed by PARET code. Axial temperature profile at the fuel centreline, fuel surface and coolant in the hot channel were generated. Fuel surface heat flux, heat transfer coefficient and Reynolds’s number for the hot channel were also calculated. The effect of fuel-cladding gap and the influence of fuel rod spacing were investigated to validate the performance of NCTRIGA code. The investigated results were found to be in good agreement with the experimental values, which indicates that the NCTRIGA code can be used with confidence for TRIGA reactor analysis.