Decrease In Heat Transfer Coefficient Research Articles

ЕКСПЕРИМЕНТАЛЬНІ ДОСЛІДЖЕННЯ МОЖЛИВОСТЕЙ ІНТЕНСИФІКАЦІЇ ТЕПЛООБМІНУ ПРИ ПАРОУТВОРЕННІ НА МІКРОКАНАЛЬНИХ ПОВЕРХНЯХ

The use of grooves, as the structure of flat plate heat pipe is widespread and is a popular and effective technical solution in the practical implementation of various design systems for thermal control of electronic equipment. Advanced thermal control system must meet two criteria: 1) have a high heat transfer coefficient; 2) remove heat of high density. However, the last criterion leads to the fact that the high density heat flux may lead to a boiling limit, which leads to a drastic decrease in heat transfer coefficient, and conducts to a rapid increase of surface temperature. The paper presents an experimental investigation of the heat transfer characteristics of the microgrooved structures at vaporization of water and ethanol. The structures are represented in the form of axial grooves with a square section dimensions of 0.3, 0.5 and 0.7 mm, and a rectangular cross section with the same width and pitch parameters of the channel but with double depth of channels. Studies showed that in the grooved structures three modes of vaporization occur as follows: evaporation, evaporation-pulsation and boiling. The experiment revealed the area having constancy of the heat transfer coefficient at varying specific heat flux that is typical for evaporation. At certain heat flux value the nucleate boiling in the grooves is initiated, and heat transfer coefficient is growing at increasing heat flux. Boiling in the channels of micro-grooved surface is accompanied by a slug boiling regime, and formation of a thin liquid film on the surface of the channel. It is essentially increasing the heat transfer characteristics of the micro-grooved structures. Double of the channel depth leads to increase in heat transfer coefficient up to 30 - 40% for both water and ethanol. Increase of the heat transfer coefficient could be explained by the enlargement of the heat transfer surface of the channels. Increasing of the effective pore radius by the increasing of the channels depth leads to reduction in the hydraulic resistance of the channels that accompanies increase of the critical heat flux up to 50 - 60%.

Numerical investigation of mist flow regime in a vertical tube

In this research, the mist flow regime, consisting dispersed water droplets in vapor flow, is simulated numerically in a vertical tube; using a discrete phase model (DPM). In this method, in addition to solving transport equations for continuous phase, a discrete phase is simulated in a Lagrangian approach and the coupling between phases is modeled through interaction terms in the transport equations. The aims of this research are to investigate thermal equilibrium and evaluate the heat transfer coefficient of mist flow regime. The results show, when thermodynamic non-equilibrium takes happen, the rate of heat transfer from the vapor to the droplets is too slow that their presence is ignored, therefore by increasing the water mass flow rate in this case, due to reduction of vapor mass flow rate, the heat transfer coefficient decreases. But when complete thermodynamic equilibrium condition is established, the rate of heat transfer from the vapor to the droplets is too fast that the vapor temperature remains at the saturation temperature until all the droplets have been evaporated and by increasing the water mass flow rate, the heat transfer coefficient will increase. In order to simulate mist flow regime in thermal equilibrium and non-equilibrium conditions, water droplets with two different diameters are injected into steam flow. The numerical results of heat transfer coefficient and wall temperature in four different vapor qualities are investigated in each state, which show good agreement with experimental data and correlations.

Experimental study of natural convection in an annulus between two eccentric horizontal square ducts

Free convection in an open-ended horizontal eccentric annulus between two eccentric horizontal square ducts with different eccentricities has been experimentally studied with several heat fluxes. Constant heat flux for outer duct and insulated inner square rod boundary conditions were used. The wall temperature of the outer duct was measured along the length for different eccentricities. The eccentricity ratio was changed (E=−1, −0.5, 0, 0.5, 1) in both upper (negative eccentricity) and lower direction (positive eccentricity). Results indicate that with the increase of eccentricity in both directions, the convection heat transfer coefficient decreases. Also, the lower heat transfer rate is found to be at the middle section between both ends led to maximum surface temperature. The generated Rayleigh number due to the heat flux was changed from 4×105 to 2.9×106. Empirical relations are derived from the average Nusselt number as a function of the Rayleigh number and the eccentricity ratio.

Heat Flux Controlled Pool Boiling of Zirconia–Water and Silver–Water Nanofluids on a Flat Plate: A Coupled Map Lattice Simulation

In the present work, the characteristic atmospheric saturated heat flux controlled pool boiling curves for zirconia–water and silver–water nanofluids have been reproduced by the coupled map lattice (CML) method using a two-dimensional (2D) boiling field model. The heater is a long horizontal flat plate of thickness 0.44 mm. The pool height is 0.7 mm. The stirring action of the bubbles is modeled by increasing the fluid thermal diffusivity by an enhancement factor. The thermal conduction in the plate is also incorporated into the model. The basic advantage of CML is that individual bubbles are not tracked, and yet the effects of bubbles are reflected qualitatively in the final solution. In the simulation of atmospheric saturated pool boiling of water minimum cavity diameter taken is 0.8 μm based on which a random distribution of cavity sizes has been specified. In the boiling of ZrO2–water nanofluid there is a deposition of nanoparticles in the cavities on the heated surface resulting in reduction of surface roughness. This feature is taken care of by proportionate decrease in minimum cavity diameter. The CML model predicts decrease in heat transfer coefficient and increase in critical heat flux (CHF) with increase in zirconia nanoparticle concentration. In the case of Ag–water nanofluid no such deposition of nanoparticles has been reported; rather surface oxidation occurs which increases the surface roughness. This is simulated by proportionately increasing the minimum cavity diameter with weight fractions of nanoparticles. The present CML model predicts increase in the heat transfer coefficient and decrease in CHF with increase in silver nanoparticle concentration. Thus, the CML results for the boiling of the aforesaid two nanofluids match qualitatively with the published experimental works.

Pool Boiling Heat Transfer Using Nanofluids

Nucleate pool boiling regime can be considered as one of the most effective ways to make a viable great amount of heat exchange in a relatively small area. To investigate the characteristics of HTC (Heat Transfer Coefficient) enhancement using nanofluids, pool boiling HTC experiments of two water – based nanofluids with alumina Al2O3and titanium TiO2 were performed using electrically heated flat plate and heating element made of stainless steel under atmospheric pressure.Systematic experiments were carried out with pure water and nanofluids containing, Al2O3 and TiO2 nanoparticles in different concentrations of (0.05w %, 0.1w %, 0.3 w%, and 0.5 w %). A comparison is made between nucleate boiling of pure water and a widely used correlation proposed in 1952 by Rohsenow is done. The results show good correspondence. Pool boiling heat transfer coefficient and phenomena of nanofluids are compared with those of pure water. The experimental results show increase in the heat transfer coefficient value and decrease in the surface superheat temperatures of heating element. This value increases with increasing nanoparticles concentration. The best nucleate boiling heat transfer performance enhancement is generally observed to be at Al2O3 nanofluid, compared to that of TiO2nanofluid and pure water.

Numerical study of cooling heat transfer of supercritical carbon dioxide in a horizontal helically coiled tube

In the present study, cooling heat transfer of supercritical CO2 in a horizontal helically coiled-tube 4 mm in diameter, 2000 mm in length, a pitch of 10 mm and 0.1 in curvature is numerically investigated with RNG turbulence model. Influences of mass flow rate, heat flux and pressure on heat transfer of supercritical CO2 are studied. The characteristics of the flow and heat transfer are compared with those in a horizontal straight tube. Results show that the secondary flow and heat transfer coefficient in a helically coiled tube is obviously larger than in a horizontal straight tube. The heat transfer coefficient of supercritical CO2 increases with increasing mass flow rate, and the heat transfer coefficient increases slightly as the heat flux increases in the gas-like region, while the heat transfer coefficient is unaffected by heat flux in the liquid-like region. The peak of the heat transfer coefficient decreases and shifts to a higher temperature region as the pressure increases.

Heat transfer coefficient calculated using a linear pressure gradient assumption and measurement for flow boiling in microchannels

Abstract To obtain local flow boiling heat transfer coefficients, some studies used a linear pressure gradient method. In this study, this method is compared with a pressure gradient measurement method. The measurement was carried out for flow boiling of de-ionized water in single horizontal microchannels. Three single microchannels with widths of 0.5 mm, 1 mm and 1.71 mm and a depth of 0.39 mm, giving hydraulic diameters of 0.438 mm, 0.561 mm and 0.635 mm, were tested. The channel length was the same, 62 mm. The nominal mass flux used was 800 kg/m2 s and heat fluxes ranging from 222 to 685 kW/m2 were applied. An inlet fluid temperature of 98 °C and an inlet pressure of 125 kPa (abs) corresponding to an inlet subcooling of 7 K, were maintained at the channel entrance. The trends of heat transfer coefficient decrease with quality at a fixed heat flux and mass flux along the channel. However, the heat transfer mechanism was dominated by nucleate boiling for the 0.561 mm and 0.635 mm channels indicated by the increased local heat transfer coefficient with heat flux, while for the 0.438 mm channel, it was dominated by convective boiling because there was no effect of heat flux on the local heat transfer coefficient. Local heat transfer coefficients determined using the two methods were found to be significantly different, especially at high heat fluxes. When the Φ is less than 9 W/m2 Pa for the 0.438 mm channel, less than 11 W/m2 Pa for the 0.561 mm channel and less than 16 W/m2 Pa for the 0.635 mm channel, the linear pressure distribution may be used with caution.

Convective Heat Transfer and Contact Resistances Effects on Performance of Conventional and Composite Thermoelectric Devices

The performance of Π shaped conventional and composite thermoelectric devices (TEDs) applied to waste heat recovery by taking the Fourier heat conduction, Joule heating, and the Peltier and Thomson effects in TE materials is investigated using analytical solutions. The TE legs built with semiconductor materials bonded onto a highly conductive interconnector material in a segmented fashion is treated as the composite TED, whereas the legs merely made from semiconductors is treated as the conventional TED. The top and bottom surfaces of TEDs are subjected to convective heat transfer conditions while the remaining surfaces exposed to ambient are kept adiabatic. The effects of contact resistances, convective heat transfer coefficients, and TE leg heights L on TEDs' performance are studied. An increase in electrical and/or thermal contact resistance and a decrease in heat transfer coefficients are resulted in a decrease in power output P0 and conversion efficiency η. Depending on the contact resistances and convective heat transfer loads, the optimum L where a maximum Po occurs is obtained typically in the range of 1–4 mm. For TE leg size greater than optimum L and TED operating under higher convective heat transfer conditions, the composite design exhibited better power output and lower conversion efficiency compared to conventional design. The effects of interconnector lengths and cross-sectional area on the composite TED's characteristics are also investigated. An increase in a length and a decrease in a cross-sectional area of the interconnector decreases the composite TED's performance. However, based on the increase of the interconnector's electrical resistance in relation to the device's total internal resistance, the composite TED exhibited both negligible and significant change behavior in P0.

Analyses on Effect of Helix Angle on the Continuous Helical Baffles Heat Exchanger

A numerical simulation for heat exchanger with continuous helical baffles was carried out. The study focuses on the effects of helix angle on heat transfer characteristics. The results show that both the shell-side heat transfer coefficient and pressure drop decrease with the increase of the helix angle at certain mass flow rate. The latter decreases more quickly than the former. The tangential velocity distribution on shell-side cross section is more uniform with continuous helical baffles than with segmental baffles. The axial velocity at certain radial position decreases as the helix angle increases in the inner region near the central dummy tube, whereas it increases as the helix angle increases in the outer region near the shell. The heat exchange quantity distribution in tubes at different radial positions is more uniform at larger helix angel.

Transient volumetric heat transfer coefficient prediction of a three-phase direct contact condenser

An experimental investigation for the time dependent volumetric heat transfer coefficient of the bubbles type, three-phase direct contact condenser has been carried out utilising a short column (70 cm in total height and 4 cm inner diameter). A 47 cm active height was chosen with five different mass flow rate ratios and three different initial dispersed phase temperatures. Vapour pentane and constant temperature tap water as dispersed and continuous phases were implemented. The results showed that the volumetric heat transfer coefficient decreases with increased time until it almost reaches its steady state conditions. A sharp decrease in the volumetric heat transfer coefficient was found at the beginning of the operation and, diminished over a short time interval. Furthermore, a positive effect of the mass flow rate ratios on the volumetric heat transfer coefficient was noted and this was more pronounced at the beginning of the operation. On the other hand, the volumetric heat transfer coefficient decreased with an increase in the continuous phase mass flow rate and there was no considerable effect of the initial dispersed phase temperatures, which confirms that latent heat transfer is dominant in the process.

Influence of pressure and surface roughness on the heat transfer efficiency during water spray quenching of 6082 aluminum alloy

The heat flux (q) and heat transfer coefficient (h) at the interface between hot aluminum surface and spray water were determined by using an inverse heat conduction method. Good agreements between numerically calculated temperatures with the inverse identified h and experimentally measurements demonstrate that the method is valid for solving the q and h of spray quenching process. The estimated heat flux consists of three main stages of transition boiling, nucleate boiling and single-phase cooling. The results show that both the heat flux and heat transfer coefficient increase with the increasing of spray pressure. When the surface temperature is lower than 170°C, the q, h and the maximum heat transfer coefficient (hmax) decrease and then increase as surface roughness increases. However, when the surface temperature is higher than 170°C, the influence of surface is insignificant. This phenomenon may be attributed to the variation of nucleation site density with surface roughness.

Influence of oil on heat transfer and pressure drop during flow boiling of CO2 at low temperatures

First experimental results for flow boiling heat transfer and pressure drop of CO2–oil mixtures at low temperatures inside a horizontal smooth tube of 14mm inner diameter are presented. For the experiments, the nominal concentration of the fully miscible lubrication oil (0–3wt%), saturation pressure (14.3–26.4bar), mass flux (75–300kgm−2s−1), inlet vapor quality (0.1–0.9) and heat flux (0–100kWm−2) were varied. Measurements were carried out inside a 14mm smooth tube under near isothermal wall condition. This was realized using a test section that consists of 6 independent electrically heated segments around the perimeter. Pressure drops and local heat transfer coefficients are presented. Results for CO2–oil mixtures reveal a significant increase in pressure drop with increasing local oil concentration (i.e. nominal oil concentration and vapor quality) and heat flux compared to measurements with pure CO2. Heat transfer coefficients tend to decrease with increasing oil concentration. As local measurements depict, the addition of oil can lead to a significant decrease in liquid heat transfer coefficient (i.e. bottom tube segments). However, local heat transfer coefficients at the top of the tube can also be enhanced due to an increase in tube wetting induced by foam formation.

Non-similar solutions for unsteady flow over a yawed cylinder with non-uniform mass transfer through a slot

Non-similar solutions are found numerically to a system of coupled non-linear partial differential equations indicating, unsteady laminar water boundary layer flow over yawed cylinder using implicit finite difference scheme along with Quasi-linearization technique. The fluid properties such as viscosity and Prandtl number are considered as an inverse function of temperature. Unsteadiness is caused by upstream velocity in and directions and non-uniform mass transfer (suction/injection) which is applied through slot on the surface of the geometry. The effect of yaw angle, variable fluid properties and non-uniform mass transfer on skin friction and heat transfer coefficients is analyzed. It is found that non-uniform slot suction and downstream movement of the slot cause the point of vanishing skin friction moves downstream, but non-uniform slot injection produces the opposite result of that corresponding to the suction case. When the yaw angle increases, both the skin friction coefficient in the – direction and the heat transfer coefficient decrease but the skin friction coefficient in the – direction increases for all times. The effect of the yaw angle is very little on the point of vanishing skin friction.

Effect of Floor Geometry on Building Heat Loss Via the Ground

This paper analyzes the heat loss from an insulated slab on the ground, focusing on the influence of floor geometry on thermal processes in the ground. The calculation model includes the vertical and horizontal structures of the building; the foundation is also included. A building with a rectangular floor is considered; the ratio between the sides of floor (defined as aspect ratio) changes from 0 to 1. The thermal analysis is carried out resorting to a finite element code, validated in accordance with the requirements of International Standard ISO 10211. Numerical results show that floor geometry has a significant influence on steady-state ground global heat transfer coefficient; ranging from a narrow rectangular floor to a square one the steady-state ground global heat transfer coefficient decreases by about 15%. The effects of the perimeter insulation are also investigated; depending on the insulating layer thickness, the decrease of the heat transfer coefficient ranges from 8% to 13%. A comparison with the results obtained by applying the International Standard method ISO 13370 is also presented.

Measuring the human body's microclimate using a thermal manikin

The human body is surrounded by a microclimate, which results from its convective release of heat. In this study, the air temperature and flow velocity of this microclimate were measured in a climate chamber at various room temperatures, using a thermal manikin simulating the heat release of the human being. Different techniques (Particle Streak Tracking, thermography, anemometry, and thermistors) were used for measurement and visualization. The manikin surface temperature was adjusted to the particular indoor climate based on simulations with a thermoregulation model (UCBerkeley Thermal Comfort Model). We found that generally, the microclimate is thinner at the lower part of the torso, but expands going up. At the head, there is a relatively thick thermal layer, which results in an ascending plume above the head. However, the microclimate shape strongly depends not only on the body segment, but also on boundary conditions: The higher the temperature difference between the surface temperature of the manikin and the air temperature, the faster the airflow in the microclimate. Finally, convective heat transfer coefficients strongly increase with falling room temperature, while radiative heat transfer coefficients decrease. The type of body segment strongly influences the convective heat transfer coefficient, while only minimally influencing the radiative heat transfer coefficient. The findings of this study generate a better understanding of the human body’s microclimate, which is important in fields such as thermal comfort, HVAC, or indoor air quality. Additionally, the measurements can be used by CFD users for the validation of their simulations.

On post-dryout heat transfer in channels with flow obstacles

This paper describes a new approach to predict post-dryout heat transfer in channels with flow obstacles. Using experimental data obtained in annular test sections at prototypical BWR conditions, the existing Saha correlation for post-dryout heat transfer has been modified to account for the presence of obstacles. The obstacle effect is taken into account in the following way: (a) the critical quality downstream of an obstacle is found; (b) an exponential function of equilibrium quality is applied to describe the decrease of heat transfer coefficient in the developing post-dryout region; (c) an additional heat transfer enhancement term is applied downstream of the obstacle. The new approach is applied to annular test sections with various flow obstacles and a significant improvement of accuracy of wall temperature prediction, as compared to reference methods, is shown.

Experimental Investigations of Pool Boiling Heat Transfer Characteristics on a Vertical Surface Using CuO Nanoparticles in Distilled Water

Experiments were conducted on pool boiling heat transfer using dilute dispersions of CuO nanoparticles in distilled water at and above atmospheric pressure. Pool boiling characteristics of CuO nanofluid were studied at different pressures and concentrations. Characterization of the heating surface was done both qualitatively and quantitatively by taking the scanning electron microscopy (SEM) images and by subsequent measurement of surface roughness of the heater. SEM images of the heater surface showed nanoparticle deposition on the heater surface, suggesting surface modification. Thorough visualization showed microcavities on the heater surface, which provide an excellent location for nucleation sites enhancing heat transfer. However, these microcavities, once filled up with the suspended nanoparticles, reduced active nucleation sites, deteriorating the boiling heat transfer coefficient. Based on the experimental investigations it was concluded that there is an optimum thickness of nanoparticles coating at which heat flux is maximum and beyond this coating boiling heat transfer coefficient decreases. At higher pressures, boiling heat transfer coefficient and specific excess temperature remained nearly the same. This showed that pressure has negligible or no role to play in boiling heat transfer using nanofluids.

FLOOR SHAPE EFFECTS ON HEAT LOSSES TO THE GROUND

In this paper, the effect of the floor shape on the steady-state heat flow to the ground of an insulated slab on the ground floor is investigated. A rectangular floor, a square one, and L- and H-shaped floors are considered. The thermal processes in the ground are analyzed by means of a finite element code, validated in accordance with the requirements of International Standard ISO 10211. The results of numerical analysis are compared with the results obtained by the International Standard method ISO 13370. The results show that the heat losses to the ground are significantly influenced by the floor shape: ranging from an H-shaped floor to a square one the steady-state ground global heat transfer coefficient decreases by about 22%. The analysis also highlights that the methodology provided by the International Standard overestimates the losses compared to numerical calculations.

Heat transfer coefficient determination for flow boiling in vertical and horizontal minichannels

The paper presents the results of boiling heat transfer research during FC-72 laminar flow along a minichannel of 1 mm depth, positioned vertically and horizontally, with an enhanced heating surface. One glass pane allows to determine the temperature of the heating wall by liquid crystal thermography. Calculations are aimed at the evaluation of one- and two-dimensional heat transfer approaches to determine the local heat transfer coefficient. In the one-dimensional approach only the direction of the flow in the channel is considered. In the two-dimensional approach the inverse problem in the heating wall and the direct problem in the glass barrier were solved by the finite element method with Trefftz functions as shape functions (FEMT). The developed flow boiling area was studied. Heat transfer coefficient values obtained for the horizontal minichannel were higher than those obtained for the vertical one. When the heat flux supplied to heating wall grows, the share of gas-phase increases leading to the heat transfer coefficient decreases. The same courses of the experiment were observed for the two applied methods, but the results obtained in the one-dimensional approach are considerably higher than in the two-dimensional one. One-dimensional approach seems to be less sensitive to measurement errors.

Condensation heat transfer in smooth inclined tubes for R134a at different saturation temperatures

This paper presents the effects of saturation temperature and inclination angle on convective heat transfer during condensation of R134a in an inclined smooth tube of inner diameter of 8.38mm. Experiments were conducted for inclination angles ranging from −90° (vertical downwards) to +90° (vertical upwards) for mass fluxes between 100kg/m2s and 400kg/m2s and mean vapour qualities between 0.1 and 0.9 for saturation temperatures ranging between 30°C and 50°C in a test matrix of 637 test data points. The results showed that saturation temperature and inclination angle strongly influenced the heat transfer coefficient. With respect to saturation temperature, an increase in saturation temperature generally led to a decrease in heat transfer coefficient irrespective of the inclination angle. The effect of inclination angle was found to be more pronounced at mass fluxes of 100kg/m2s and 200kg/m2s for the range of mean vapour qualities considered. Within the region of influence of inclination, there was an optimum angle, which was between −15° and −30° (downward flow). The inclination effect corresponded to the predominance of the effect of gravity on the flow distribution. An increase in saturation temperature increased the effect of inclination on two-phase heat transfer.