Decrease In Heat Transfer Coefficient Research Articles

Determination of the heat transfer coefficient from short‐shots studies and precise simulation of microinjection molding

AbstractThe filling process of a micro‐cavity was analyzed by modeling the compressible filling stage by using pressure‐dependent viscosity and adjusted heat transfer coefficients. Experimental filling studies were carried out at the same time on an accurately controlled microinjection molding machine. On the basis of the relationship between the injection pressure and the filling degree, essential factors for the quality of the simulation can be identified. It can be shown that the flow behavior of the melt in a micro‐cavity with a high aspect ratio is extremely dependent on the melt compressibility in the injection cylinder. This phenomenon needs to be considered in the simulation to predict an accurate flow rate. The heat transfer coefficient between the melt and the mold wall that was determined by the reverse engineering varies significantly even during the filling stage. With increasing injection speed and increasing cavity thickness, the heat transfer coefficient decreases. It is believed that the level of the cavity pressure is responsible for the resulting heat transfer between the polymer and the mold. A pressure‐dependent model for the heat transfer coefficient would be able to significantly improve the quality of the process simulation. POLYM. ENG. SCI., 2010. © 2009 Society of Plastics Engineers

Long-term performance of air-side heat transfer and pressure drop for finned tube evaporators of air conditioners under intermittent operation conditions

In this study, the effects of long-term intermittent operations on the air-side heat transfer and pressure drop performance of finned tube evaporators of air conditioners were investigated by experiments on an aluminum-fin evaporator and a copper-fin evaporator. In order to simulate intermittent operations of on–off controlled air conditioners, the temperatures of the two evaporators changed to 5 ± 0.5 °C at first and then to 27 ± 0.5 °C repeatedly. The repetition number was up to 4800, and the air-side heat transfer and pressure drop of the two evaporators were tested after every 300 repetitions. The test results indicate that after long-term intermittent operations, the air-side heat transfer coefficient decreases and the pressure drop increases. The variations of the heat transfer coefficient and the pressure drop are more obvious at lower inlet air velocity, and the influence of long-term intermittent operations on the aluminum-fin evaporator is greater than that on the copper-fin evaporator.

CFD modelling of transparent bubble cavity envelopes for energy efficient greenhouses

Traditional greenhouses with single glazing have very poor thermal performance and the energy use for providing a productive internal environment is high. A cavity envelope formed by two transparent membranes and filled with transparent liquid bubbles can provide the greenhouse with super thermal performance. The liquid bubbles in the cavity transmit daylight while providing a thick blanket of insulation, thus reducing heat losses in winter and solar gains in summer. A computer model has been developed to simulate the thermal performance of a transparent bubble cavity envelope and the internal environment of the greenhouse. It has been found that the cavity convective heat transfer coefficient, equivalent thermal conductivity and thermal transmittance of a transparent bubble cavity envelope increase with bubble diameter and porosity as well as the internal–external temperature difference when bubbles are not compact in the cavity. The equivalent thermal conductivity and thermal resistance also increase with cavity width but the cavity convective heat transfer coefficient and thermal transmittance decrease with the increase in cavity width. A greenhouse with the transparent bubble cavity envelope has a better thermal environment with a significantly less energy requirement than does a traditional greenhouse.

Effect of Mass Flow Rate on the Convective Heat Transfer Coefficient: Analysis for Constant Velocity and Constant Area Case

The effect of the mass flow rate at constant velocity on the convective heat transfer coefficient of an incompressible fluid in a turbulent flow regime is presented with the help of dimensional analysis. The heat transfer coefficient decreases by ˜10% with a threefold increase in the mass flow rate under these conditions, based on the commonly used Dittus-Boelter correlation for estimation of the heat transfer coefficient. On the other hand, an increase in the heat transfer coefficient is observed if the area is maintained constant. Doubling the mass flow rate will result in a 92% increase in the heat transfer coefficient. However, there is a concomitant increase in the pressure drop, proportional to the mass flow rate raised to 0.95. The pressure drop is predicted to decrease for the constant velocity case with an inverse dependence on the mass flow rate. The pressure drop considerations may be critical in certain situations (elevation of boiling point in case of a boiling heat transfer medium), and any benefit derived from the higher heat transfer coefficient may be lost because of the higher pressure drop across the heat exchanger in the constant area case.

An experimental study of a confined and submerged impinging jet heat transfer using Al 2O 3-water nanofluid

An experimental investigation was performed to study the heat transfer performance of a 36 nm-Al 2O 3-particle–water nanofluid in a confined and submerged impinging jet on a flat, horizontal and circular heated surface. The tests were realized for the following ranges of the governing parameters: the nozzle diameter is 3 mm and the distance nozzle-to-heated-surface was set to 2, 5 and 10 mm; the flow Reynolds number varies from 3800 to 88 000, the Prandtl number from 5 to 10, and the particle volume fraction is ranging from 0 to 6%. Experimental data, obtained for both laminar and turbulent flow regimes, have clearly shown that, depending upon the combination of nozzle-to-heated surface distance and particle volume fraction, the use of a nanofluid can provide a heat transfer enhancement in some cases; conversely, for other combinations, an adverse effect on the convective heat transfer coefficient may occur. Within the experimental parameters used, it has been observed that highest surface heat transfer coefficients can be achieved using an intermediate nozzle-to-surface distance of 5 mm and a 2.8% particle volume fraction nanofluid. Nanofluids with high particle volume fractions, say 6% or higher, have been found not appropriate for the heat transfer enhancement purpose under the confined impinging jet configuration. On the other hand, for a very small and a large distance of nozzle-to-heated-surface, it has been observed that the nanofluid use does not provide a perceptible heat transfer enhancement and has, for some particular cases, produced a clear decrease of the convective heat transfer coefficient while compared to that obtained using distilled water.

Natural convective heat transfer in vertical channels with low-thermal-conductivity ribs

The effect of repeated horizontal protrusions on the free-convection heat transfer in a vertical, asymmetrically heated, channel has been experimentally investigated. The protrusions have a square section and are made of a low-thermal-conductivity material. Experiments were conducted by varying the number of the protrusions over the heated surface (whose height was held fixed) and the aspect ratio of the channel. The convective fluid was air and the wall-to-ambient air temperature difference was set equal to 45 K. The local heat transfer coefficient was obtained by means of the schlieren optical technique. The protrusions were found to significantly alter the heat transfer distribution along the heated surface of the channel, especially in the vicinity of each obstacle. For the ranges of parameters studied, the addition of low-conductivity protrusions leads to a decrease in the average heat transfer coefficient, as compared to that for the smooth surface, in the 0–7% range for the largest channel aspect ratio and in the 18–43% for the smallest channel aspect ratio.

Effect of boundary layer thickness before the flow separation on aerodynamic characteristics and heat transfer behind an abrupt expansion in a round tube

Results of numerical investigation of the boundary layer thickness on turbulent separation and heat transfer in a tube with an abrupt expansion are shown. The Menter turbulence model of shear stress transfer implemented in Fluent package was used for calculations. The range of Reynolds numbers was from 5·103 to 105. The air was used as the working fluid. A degree of tube expansion was (D2/D1)2 = 1.78. A significant effect of thickness of the separated boundary layer both on dynamic and thermal characteristics of the flow is shown. In particular, it was found that with an increase in the boundary layer thickness the recirculation zone increases, and the maximum heat transfer coefficient decreases.

Effect of Mold Coating Materials and Thickness on Heat Transfer in Permanent Mold Casting of Aluminum Alloys

In permanent mold casting or gravity die casting (GDC) of aluminum alloys, die coating at the casting-mold interface is the most important single factor controlling heat transfer and, hence, it has the greatest influence on the solidification rate and development of microstructure. This investigation studies the influence of coating thickness, coating composition, and alloy composition on the heat transfer at the casting-mold interface. Both graphite and TiO2-based coatings have been investigated. Two aluminum alloys have been investigated: Al-7Si-0.3Mg and Al-9Si-3Cu. Thermal histories throughout the die wall have been recorded by fine type-K thermocouples. From these measurements, die surface temperatures and heat flux density have been evaluated using an inverse method. Casting surface temperature was measured by infrared pyrometry, and the interfacial heat-transfer coefficient (HTC) has been determined using these combined pieces of information. While the alloy is liquid, the coating material has only a weak influence over heat flow and the thermal contact resistance seems to be governed more by coating porosity and thickness. The HTC decreases as the coating thickness increases. However, as solidification takes place and the HTC decreases, the HTC of graphite coating remains higher than that of ceramic coatings of similar thickness. After the formation of an air gap at the interface, the effect of coating material vanishes. The peak values of HTC and the heat flux density are larger for Al-7Si-0.3Mg than for Al-9Si-3Cu. Consequently, the apparent solidification time of Al-9Si-3Cu is larger than that of Al-7Si-0.3Mg and it increases with coating thickness.

Condensation heat transfer coefficients of HCFC22, R410A, R407C and HFC134a at various temperatures on a plain horizontal tube

In this study, external condensation heat transfer coefficients (HTCs) of HCFC22, R410A, R407C, and HFC134a were measured on a smooth horizontal tube at 30, 39, and 50℃ with the wall subcooling of 3-8℃. The results showed that condensation HTCs decreased for all fluids tested with an increase in temperature. This is due mainly to such properties as the saturated liquid density and liquid thermal conductivity. These properties decrease as the temperature increase and accordingly HTCs decrease. The condensation HTCs of R410A are 9.2~19.7% higher than those of HCFC22 while those of R134a are 2.5~10.2% lower than those of HCFC22. Condensation HTCs of R407C, nonazeotropic mixture, are 29.4~34.3% lower than those of HCFC22. Overall, the HTCs of R407C are much lower than those of HCFC22, HFC134a and R410A due to the mass transfer resistance in a diffusion vapor film. Condensation HTCs of HCFC22 and HFC134a are higher than those calculated by Nusselt’s equation by 7.7~11.8% and 4.0~11.1% respectively. On the other hand, HTCs of R407C measured on plain tube, however, are not well predicted by these well-known prediction correlations due to the introduction of mass transfer resistance associated with non-azeotropic mixtures.

Influence of a pulsation on heat transfer and flow structure in submerged impinging jets

An experimental investigation on pulsating impinging jets has been performed. The effect of the pulsation on the flow structure and heat transfer have been investigated. Frequency and amplitude were varied separately and the effect of each parameter was examined for different Reynolds numbers and nozzle-to-plate distances. The jet was found to become broader and the core jet length smaller with the pulsation. The reason for this behavior is that pulsation enhanced entrainment of air into the jet, which results in a change of mean velocity of the jet. Nevertheless, the behavior at lower frequencies (up to 140 Hz) is still quasisteady. This means that the amplitude of the pulsation behaves similar to the mean velocity of the jet, that the shapes of the velocity profiles are comparable to steady jets and that the jet behavior is independent of frequency. At moderate frequencies heat transfer is only affected by the pulsation when nozzle-to-plate distance and amplitude are large enough. At small nozzle-to-plate distances enhanced entrainment has no influence and no difference between steady and pulsating jets can be recognized. At large nozzle-to-plate distances entrainment increases and jet velocity reduces. This yields a reduction of heat transfer in the stagnation point of up to 50%. But besides of this effect of enhanced entrainment a theoretical limit could be determined, above which the jet is not anymore quasisteady. Above Sr = 0.2 heat transfer is affected by the pulsation also at small nozzle-to-plate distances. At this frequency boundary layer is also affected by the pulsation. This yields increased heat transfer coefficients at the stagnation point. For larger nozzle-to-plate spacings this effect is superposed by the reduction of heat transfer due to increased entrainment, resulting in a strong decrease of heat transfer coefficient.

Formulation of nanofluids for natural convective heat transfer applications

The paper is concerned about formulation of aqueous based nanofluids and its application under natural convective heat transfer conditions. Titanium dioxide nanoparticles are dispersed in distilled water through electrostatic stabilization mechanisms and with the aid of a high shear mixing homogenizer. Nanofluids formulated in such a way are found very stable and are used to investigate their heat transfer behaviour under the natural convection conditions. The preliminary results are presented in this paper. Both transient and steady heat transfer coefficients are measured and the results show a systematic decrease in the natural convective heat transfer coefficient with increasing particle concentration. This is in contradiction to the initial expectation. Possible reasons for the observations are discussed.

Solid effects on hydrodynamics and heat transfer in an external loop airlift reactor

The effect of a solid presence on global hydrodynamic parameters and heat transfer in an external loop airlift reactor has been experimentally investigated. Results obtained in both two- and three-phase flow are presented in this study. Two different external loop airlift reactor sizes have been used and local hydrodynamic characteristics including local gas hold-up and bubble velocity have been obtained in two-phase flow. Optical and ultrasound probes have been used to obtain this information, respectively. It was found that an increase of solid hold-up leads to a decrease of liquid velocity and heat transfer coefficient. Measured in a two- and three-phase reactor using a horizontal-heating probe, a correlation of the average gas hold-up and heat transfer coefficient is proposed. Correlation parameters are identified in homogeneous and heterogeneous flow regimes, which have been derived from the gas slip velocity concept. The experimental liquid velocity and gas hold-up in the riser have been represented in a satisfactory way by a hydrodynamic model, either in the absence or in the presence of solid particles.

Effect of noncondensable gas in a vertical tube condenser

An experimental study is performed to investigate the effects of noncondensable (NC) gas in the steam condensing system. A vertical condenser tube is submerged in a water pool where the heat from the condenser tube is removed by boiling heat transfer. The design of the test section is based on the passive condenser system in an advanced boiling water nuclear power reactor. Data are obtained for various process parameters, such as inlet steam flow rate, noncondensable gas concentration, and system pressure. Degradation of the condensing performance with increasing noncondensable gas is investigated. The condensation heat transfer coefficient and heat transfer rate decrease with noncondensable gas. The condensation heat transfer rate is enhanced by increasing the inlet steam flow rate and the pressure. The condensation heat transfer coefficient increases with the inlet steam flow rate, however, decreases with the system pressure. For the condenser submerged in a water pool with saturated condition, the strong primary pressure dependency is observed.

Air side performance at low Reynolds number of cross-flow heat exchanger using crimped spiral fins

A total of 23 cross-flow heat exchangers having crimped spiral configurations is studied. The effect of tube diameter, fin spacing, transverse tube pitch, and tube arrangements are examined. For the inline arrangement, the pressure drop increases with the rise of tube diameter but the associated heat transfer coefficient decreases with it. The increase of fin height also gives rise to considerable increase of pressure drop and decrease of heat transfer coefficients for the inline arrangement. However, for the staggered arrangement, the effect of the fin height on the pressure drop is much smaller than that of the inline arrangement due to the major contribution to the total pressure drops from the blockage of the airflow from staggered arrangement. Effect of the fin spacing on the air side performance is strongly related to the transverse tube pitch for both inline and staggered arrangements. Correlations of the present crimped spiral fins in both staggered and inline arrangements are developed. The proposed correlations give fairly good predictive ability against the present test data.

403 スリットから吸込みを伴う管内乱流に関する研究(G05-8 乱流,G05 流体工学)

Experiments have been performed on turbulent channel flow with suction through a slit. The inclined angle of the suction is changed 30°, 60°and 90°at the main stream velocity 10m/s. The dimensionless suction flow ratio is varied nine steps in the range of 0.067〜0.500. Difference due to the inclined angle is not observed in the result of wall pressure and heat transfer coefficient. Increasing the suction flow ratio, the heat transfer coefficient decreases. The recovery mechanism of turbulence is greatly different between the lower side and upper side of the channel.

460 伝熱ダクト内流体の対流熱伝達促進と圧力低減対策に関する数値解析による研究(スリット付挿入板を設置した場合)(熱交換器,環境保全型エネルギー技術)

This present paper attempts to numerically estimate heat transfer enhancement and pressure drop decrease on a duct flow system of heat exchanger. Inserted plates inside the duct flow are proposed in order to enhance heat transfer coefficient, however the inserted plates cause a significant pressure drop, In order to decrease the pressure drop, the inserted plates with slits were employed. Numerical calculations of two-dimensional, laminar, and steady state conditions of duct flow with and without inserted plates were carried out. Air, as a heated media, with range of velocities 0.5m/s-1.5m/s and ambient temperature 20℃ was heated by using 50℃ isothermal duct wall. The calculation results show that the setting of inserted plates enhances the heat transfer coefficient significantly but pressure drop also increase. The presence of slits hole in the inserted plates decreases the pressure drop significantly but the heat transfer coefficient decrease slightly. The calculated streamlines, isotherms, vector velocity, local and average heat transfer coefficient, total heat transfer, and required fan power (pressure drop) are presented.

Are the energy conservation rates (RVRs) approximate in different cities for the same building with the same outer-wall thermal insulation measures?

This paper chooses two cities with completely different hourly weather conditions as background, for example Chicago in USA and Shanghai in China. And then, the author makes hourly and classified analysis on the influence of the decrease of outer-wall heat transfer coefficient on the heating and cooling energy consumption and its RVRs. It is found that the reduction of outer-wall heat transfer coefficient U can significantly decrease the annual heating needs and classified heating needs both with and without solar radiation; It can also limitedly reduce the annual and classified cooling needs both with and without solar radiation. The degree of cooling reduction is obviously lower than that of heating. Despite of any climatic conditions, the proportion of the heating energy consumption without solar radiation in the total heating needs ( > 80%) is far higher than that with solar radiation ( < 20%) while the proportion of the cooling energy consumption with solar radiation in the total cooling needs ( > 90%) is far higher than that without solar radiation ( < 10%). Therefore, the reduction of outer-wall's U can greatly increase the energy conservation rates (RVRs) of heating and cooling (called the basic RVRs) when there is no solar radiation. In order to reach the best energy efficiency effect, solar radiation should be utilized to the limit in winter and controlled effectively in summer. The annual energy conservation rates of heating as well as cooling are both close in different cities under the same outer-wall U , that is taking the same outer-wall thermal insulation measures to identical building. This paper proves in all aspects that the energy efficiency rates of both heating and cooling are approximate under different climatic conditions (or in various cities) when taking the same energy efficiency measures to the same building.

Experimental Study on Transient Boiling Heat Transfer in an Annulus with a Narrow Gap

The importance of heat transfer in a narrow gap has been pointed out in relation to the in-vessel retention of molten core under a severe accident condition of a light water reactor. In this study, heat transfer experiment has been carried out using internally heated annuli with a narrow gap and closed bottom. Effects of gap clearances (1.0, 2.0, 4.0 mm) on the cooling process were examined. The inner wall initially heated up to 800°C was cooled down by water supplied to the upper plenum, the transient rod temperature being measured. From the temperature history, transient boiling curves were obtained, which were compared with predictions by existing heat transfer correlations for nucleate boiling critical heat flux (CHF) and film boiling. In narrow gap channels water penetration to the gap was restricted by counter flowing vapor, which lead to a decrease of heat transfer coefficients.

Experimental Study on Transient Boiling Heat Transfer in an Annulus with a Narrow Gap

The importance of heat transfer in a narrow gap has been pointed out in relation to the in-vessel retention of molten core under a severe accident condition of a light water reactor. In this study, heat transfer experiment has been carried out using internally heated annuli with a narrow gap and closed bottom. Effects of gap clearances (1.0, 2.0, 4.0 mm) on the cooling process were examined. The inner wall initially heated up to 800°C was cooled down by water supplied to the upper plenum, the transient rod temperature being measured. From the temperature history, transient boiling curves were obtained, which were compared with predictions by existing heat transfer correlations for nucleate boiling critical heat flux (CHF) and film boiling. In narrow gap channels water penetration to the gap was restricted by counterflowing vapor, which lead to a decrease of heat transfer coefficients.

A risk based performance evaluation of plate-and-frame heat exchangers

Plate-and-frame heat exchangers (PHEs) operating in process industries are fouled to a greater or lesser extent depending on surface temperature, surface condition, material of construction, fluid velocity, flow geometry and fluid composition. This fouling phenomenon is time-dependent and will result in a decrease in the overall heat transfer coefficient and increase in the pressure drop of the PHE. Once the overall heat transfer coefficient decreases to a minimum acceptable level, cleaning of the equipment becomes necessary to restore the performance. In this paper, we present a simple probabilistic approach to characterize various fouling models that are commonly encountered in many industries. These random fouling growth models are then used to investigate the impact on risk based thermal effectiveness, overall heat transfer coefficient and the hot- and cold-fluid outlet temperatures of a PHE. All the results are presented in a generalized form in order to demonstrate the generality of the risk-based procedure discussed in this paper.