Decrease In Heat Transfer Coefficient Research Articles

Numerical modeling of thermal-fluid behavior of the S-CO2 cooled pebble bed reactor

The S-CO2 cooled pebble bed reactor (SCPBR) is a type of pebble bed reactors in which the supercritical carbon dioxide used as a coolant. In this study, the thermal-fluid phenomena in the SCPBR core, at different porosities (0.38, 0.4, and 0.42) and pebble diameters (3 cm, 8 cm and 12 cm) are numerically studied by developing a three-dimensional CFD model. ANSYS FLUENT 14.5, was used to carry out the numerical simulation. The results show that for a given pebble diameter, when the porosity is increased, the fluid and solid temperatures are almost the same, the pressure drop decreases, and the heat transfer coefficient increases. Also, it was found when the pebble diameter is increased, the fluid temperature is almost the same, the pressure drop and the heat transfer coefficient decrease, but the solid temperature increases.

Gravity effects on subcooled flow boiling heat transfer

Subcooled flow boiling measurements using HFE-7000 were obtained in a vertical 6 mm ID sapphire tube during upward and downward flow at various gravity levels including hypergravity and microgravity. Temperature sensitive paint (TSP) applied to the inside of the tube was used to measure time and space resolved temperature and heat transfer distributions at the wall–fluid interface, and this data along with flow visualization were used to characterize the heat transfer for different flow patterns. Time-averaged heat transfer coefficients were compared at nine gravity levels, four mass fluxes, six heat fluxes, and two subcoolings. The average heat transfer coefficient typically increased with heat flux, mass flux, and absolute gravity level. In microgravity, the lack of mixing at low heat fluxes due to the absence of natural convection and bubble slip velocity resulted in a decrease in heat transfer coefficient compared to downward and upward conditions. The heat transfer was strongly dependent on the flow regime, causing certain data points at high mass flux or low gravity to deviate from the typical trends due to deactivation of nucleation sites. The heat transfer coefficient became less dependent on gravity as the mass flux and heat flux increased. Flow regimes were very sensitive to the competition between buoyancy and inertial forces, which in turn affected the heat transfer. Mechanisms by which heat is transferred under various conditions are discussed.

X-ray imaging analysis on behaviors of boiling bubbles in nanofluids

Nanofluid, a liquid suspension containing nanoparticles, has been widely used to enhance heat transfer. However, the heat transfer enhancement mechanism of nanofluids has not been clearly revealed yet. Therefore, understanding the boiling heat transfer of nanofluids is a challenging research issue in the field of heat transfer. When nanoparticles are added into a base fluid, the thermo-physical properties of the fluid and surface characteristics are modified. Those modifications induce changes in the behavior of boiling bubbles. In addition to the size and number of bubbles, the generating rate of boiling bubbles was newly defined to estimate the heat transfer coefficient directly from bubble behaviors according to nanofluid concentration. As the nanofluid concentration increases, both the generating rate of boiling bubbles and the heat transfer coefficient decrease. Wettability and hydrodynamic size were also employed to reason those degradations. Wettability increase leads to the reduction of activated nucleation sites, which reduces the bubble generating rate and heat transfer coefficient with an increase in nanofluid concentration. In this study, the feasibility and usefulness of synchrotron X-ray imaging and a newly defined boiling generating rate for examining boiling heat transfer were verified by observing boiling-bubble behaviors. Moreover, it was also shown that wettability plays an important role in changes of the bubble behaviors and the heat transfer coefficient as surface roughness modification.

Condensation heat transfer characteristics of low-GWP refrigerants in a smooth horizontal mini tube

In this paper, a comprehensive study on heat transfer coefficient (HTC) and pressure drop of the two-phase condensation for R1234ze(E), R290, R161 and R41 were obtained in a smooth horizontal tube with 2 mm inner diameter, for these refrigerants being considered as environmentally friendly working fluid candidates in refrigeration and air conditioning systems. HTCs of R32 and R134a were also investigated as a baseline in this study. Condensation temperatures were from 35 °C to 45 °C. The mass flux ranged from 200 kg/m2·s to 400 kg/m2·s and the heat flux from 8 to 30 kW/m2. The results show that with the increasing saturated temperature and heat flux, the condensation heat transfer coefficients decrease, the HTCs of R161 are the highest while those of R1234ze(E) are the lowest at the same working condition. The pressure drop decreases while the saturation temperature rises. The predicted values of HTCs by 7 models were compared with experimental data. The results show that although different models are suitable for different refrigerants, Koyama et al.’s model is prevailing over other models for most refrigerants. On the basis of Koyama et al.’s model, a modified ϕv correlation with Weber number was taken into consideration and the revised model shows an improved prediction accuracy.

Nanofluids for heat transfer applications: a review

Since diluted suspensions of nanoparticles were first called nanofluids and presented as viable solutions for heat transfer applications, this subject has received much attention and related investigations have expanded to many paths. In order to comprehend how nanoscale-related effects could influence the macroscopic transport behavior of nanofluids under single or phase-change conditions, researchers have studied, for example, the stability of these solutions, variation of thermal and rheological properties, and the convective heat transfer behavior of a great variety of nanofillers in common fluids, mainly water. The deposition of nanofillers over heated surfaces has also been investigated due to the role of surface nanostructuring in modifying wettability, thermal resistance, and delaying the occurrence of critical heat flux. Despite the considerable number of publications regarding nanofluids, scattered results for transport properties or convective behavior of nanofluids under similar experimental conditions are often found, which hinders their applications due to a lack of comprehension on the mechanisms related to the behavior of these fluids and, consequently, to the difficulty in predicting it. In this context, this work concerns a review about the heat transfer behavior of nanofluids under single-phase flow, pool boiling, and flow boiling conditions. In general, there is a consensus that the heat transfer coefficient of single-phase flow is enhanced by the addition of nanoparticles to base fluids, although overall benefits of their application cannot be assured due to increases in viscosity. In contrast, either increase or decrease in heat transfer coefficient could be observed for pool and flow boiling conditions. Such behavior can be attributed to surface modifications due to interactions between the bare surface texture and the deposited nanoparticles; however, information on the surface texture is commonly missing in most works. Finally, the main mechanisms reported in the literature pointed out as responsible for the heat transfer coefficient behaviors are summarized, where it can be seen that modifications of transport properties and particles movements impact single-phase flow, while phase-change heat transfer is also influenced by variations of surface characteristics.

Nanoparticle layer detachment and its influence on the heat transfer characteristics in saturated pool boiling of nanofluids

In nucleate pool boiling of nanofluid, nanoparticles suspended in liquid are deposited to form nanoparticle layer on the heated surface. As a result, surface properties are changed and the critical heat flux (CHF) is usually enhanced. However, since adhesion of the nanoparticle layer to the heated surface is not necessarily strong, partial detachment of the nanoparticle layer frequently occurs during nucleate boiling. In this study, peeling test was conducted for the nanoparticle layer formed during nucleate boiling of the water-based nanofluids to measure the adhesion force. The material of the heated surface was copper, and TiO2, Al2O3 and SiO2 were used as the nanoparticle material. It was found that the adhesion force is highly dependent on the nanoparticle material; it was greatest for SiO2 and weakest for TiO2 in the present experiments. Then, saturated pool boiling curves were obtained using the damaged heated surfaces after the peeling. For the nanoparticle-layer coated surfaces without peeling, the heat transfer coefficient (HTC) was lower and CHF was higher than those for the bare surface. It was shown that with an increase in the mass of the nanoparticles removed by peeling, the decrease of HTC and the increase of CHF were mitigated. Even higher HTC values and a lower CHF value than those for the bare heated surface were measured for significantly damaged surfaces.

Effects of mass transfer on heat and mass transfer characteristics between water surface and airstream

A numerical study has been conducted to simulate the convective heat and mass transfer between water surface and air fluid flowing over it. The airflow is laminar and steady, and has a temperature much higher than the water, causing a combined heat and mass transfer accompanied with water evaporation into the airstream. Calculations are performed to investigate the effects of mass flux on heat and mass transfer coefficients and the applicability of the Chilton-Colburn analogy for 200 °C air temperature, 1–10 m/s air velocities, and 10–90 °C water temperatures. Calculations are implemented with and without consideration of the air property variations caused by the air temperature and humidity changes near the water surface and in the airflow direction. The results show that the heat and mass transfer coefficients both decrease with increasing water surface temperature, i.e. increasing mass flux. The Chilton-Colburn analogy holds only for low water temperature case, the deviation of the heat to mass transfer coefficient ratio given by the Chilton-Colburn analogy relative to that by the numerical simulation is less than 5% when the water surface temperature is below 60 °C. The air property variability has a notable and complex effect on heat transfer coefficient but an inconspicuous effect on mass transfer coefficient.

Validating solid wall insulation retrofits with in-use data

Improving the energy efficiency of the UK housing stock is important both to meet carbon emission reduction targets and to reduce fuel poverty. For this reason, domestic properties are frequently retrofitted with energy saving measures. This study looks at how the energy consumption, thermal properties and internal temperature of 14 dwellings change as a result of a solid wall insulation (SWI) retrofit. A decrease in heat transfer coefficient of 11+6−7% was calculated for 2 dwellings, which is slightly lower than the previously modelled value of 18%. However, many houses displayed evidence that the full benefit of SWI was not being realised as, for example, energy savings were offset with increases in internal temperature. Future retrofit schemes should therefore consider supplementing the changes in fabric with increased guidance for the occupant.

Experimental determination of the heat transfer coefficient during evaporation and boiling of thin liquid film

Thin films evaporation of distilled water, ethanol and HFE-7100 liquid was experimentally studied. The dependences of heat transfer coefficients in time were determined. It has been established that with a decrease in the layer thickness of distilled water and ethanol, the heat transfer coefficient increases. For the HFE liquid, the nature of the change is different: as the time increases, the heat transfer coefficient decreases.

Cleaning Schedule Operations in Heat Exchanger Networks

Heat exchanger networks have been known to be the essential parts in the chemical industries. Unfortunately, since the performance of heat exchanger can be decreasing in transferring the heat from hot stream into cold stream due to fouling, then cleaning the heat exchanger is needed to restore its initial performance periodically. A process of heating crude oil in a refinery plant was used as a case study. As many as eleven heat exchangers were used to heat crude oil before it was heated by a furnace to the temperature required to the crude unit distillation column. The purpose of this study is to determine the cleaning schedule of heat exchanger on the heat exchanger networks due to the decrease of the overall heat transfer coefficient by various percentage of the design value. A close study on the process of heat exchanger cleaning schedule in heat exchanger networks using the method of decreasing overall heat transfer coefficient as target. The result showed that the higher the fouling value the more often the heat exchanger is cleaned because the overall heat transfer coefficient decreases quickly.

Influences of tube pitches on heat transfer and pressure drop characteristics of two-phase propane flow boiling in shell side of LNG spiral wound heat exchanger

For clarifying the influences of tube pitches on heat transfer and pressure drop characteristics of two-phase flow in the shell side of LNG spiral wound heat exchanger, experiments are performed on test samples covering the longitude tube pitch of 2–6 mm and the radial tube pitch of 1–4 mm. Propane is used as the tested fluid. The operational conditions cover the vapor quality of 0.2–0.9, the mass flux of 40–80 kg (m2 s)−1, the heat flux of 4–10 kW m−2, the pressure of 0.25 MPa and the temperature of −19.4 °C. The uncertainties of tested heat transfer coefficients and pressure drops are within 14.4% and 4.0%, respectively. The results indicate that, as the longitude tube pitch increases, both the heat transfer coefficient and pressure drop decrease; while as the radial tube pitch increases, the heat transfer coefficient always decreases for the mass flux of 60 and 80 kg (m2 s)−1, but varies non-monotonously for the mass flux of 40 kg (m2 s)−1; and the pressure drop decreases maximally by 61% with the increasing radial tube pitch. Two-phase heat transfer and pressure drop correlations reflecting the influences of tube pitches are developed, and the deviations of the predicted heat transfer coefficients and pressure drops from the experimental data are within ± 20% and ± 25%, respectively.

INTENSIFICATION OF HEAT TRANSFER IN A HIGH-TEMPERATURED FLUIDIZED BED

Purpose. This paper highlights experimental research of heat exchange in coarse particles to ensure the performance of ecological characteristics of Heat supply system. Methodology. The test stand has been developed to solve the defined task. It helps to do the research at the temperature of fluidizing bed and pulsating fluidizing bed at the range 800–1000°С. The temperature of the fluidized bed was provided by burning natural gas and wood waste. Sand and chamotte with a particle size of 1.0 to 5.0 mm were used as the layer material. The heat-transfer coefficient from the layer to the surface, immersed in the layer, and the density of the heat flux were measured by a calorimetric method under steady-state conditions. Smooth tubes and transversely finned with different height and rib spacing were investigated. Experiments in a high-temperature pulsating fluidized bed were carried out with pulsating combustion of natural gas in a layer or in a sublattice chamber into which natural gas and air were separately supplied. The frequency of combustion pulsations was provided by the automation system. The flash frequency was regulated in the range from 0.14 to 5 Hz. Findings. It is presented the results of physical modeling to find out the heat-transfer coefficients of smooth and finned tubes in fluidizing bed of coarse particles in the process of wood waste and gaseous fuel combusting. It is proved that the coefficient of heat transfer increases with increasing temperature by 2–2,5 times in the bed which contains particles diameter of 2.5–5 mm is 300–350Vt/(m2 К) that is much higher than for layer furnaces. The results of the experiments are presented in the form of a generalized relationship that takes into account the diameter of the particles and the value of the finning coefficient. Heat transfer of finned tubes is 15–20% less then smooth tubes but the density of heat flow referred to the area of a finned tube is 0,12–0,20 МVt/m2 that is higher than for smooth ones. At pulsating fuel supply with frequency 1-2 Hz the heat-transfer coefficient gets higher from 10–15% till 25–30%. At a ripple frequency of 2 to 5 Hz, the heat-transfer coefficient varies from 510 to 570W/(m2K). With a further increase in the pulsation frequency, the heat-transfer coefficient decreases and amounts to 515–520 W/(m2K). A significant effect of the ripple pulsation from 0.8 to 0.3 is shown: the heat-transfer coefficient is increased by 30–35%. However, an unstable combustion regime of the gas-air mixture and the ejection of material particles from the layer are observed. The results of the experiments are presented in the form of a generalized relationship. Originality. The authors have presented the obtained experimental data about heat-transfer coefficients of finned tubes in fluidizing bed and pulsing fluidizing bed of coarse particles at the temperature of layer 800–1100оС. Practical value. The results obtained at the research help to develop furnace devices with low-temperature fluidizing bed in the process of immersion in a layer of finned tubes layers. The research data is presented in criterial view which is convenient for practical using.

Heat transfer enhancement in hydromagnetic dissipative flow past a moving wedge suspended by H2O-aluminum alloy nanoparticles in the presence of thermal radiation

This letter investigates the two dimensional magnetohydrodynamic Falkner Skan flow of water saturated with AA7072–H2O and AA7075–H2O nanoparticles over a moving wedge. Influence of viscous dissipation and thermal radiation is also under consideration. The model is reduced into nondimensional form by using defined self-similar transformations. Then, numerical study (Runge-Kutta numerical scheme) is carried out for solution purpose. The effects of pertinent flow parameters are discussed in the flow regimes by means of graphical aid. Graphical results for special cases (static wedge and when the movement of flow and wedge is in opposite direction) of the model are also elucidated graphically. It is noted that fluid velocity decreases for volumetric fraction and magnetic force favor the velocity. Significant effects of thermal radiation and nanoparticles volume fraction pointed for thermal filed and the influence of Eckert number is almost inconsequential. For more radiative fluid heat transfer coefficient decreases and nanoparticles volumetric fraction favor the local rate of heat transfer. In the presence of AA7075 nanoparticles, rate of heat transfer is quite rapid as compared to that of AA7072 nanoparticles. To validate the present computations, some present results are compared with already existing results in the literature and found better agreement between them. Finally, major points of the study ingrained in the last section of the letter.

Circumferential distribution of local heat transfer coefficient during steam stratified flow condensation in vacuum horizontal tube

An experimental system was built up to measure the wetted angle and circumferential distributions of local heat transfer coefficient during the steam stratified flow condensation in vacuum horizontal tube. The wetted angle is mainly affected by vapor quality. It decreases with vapor quality and steam mass flow rate, and increases with the steam saturation temperature. The Biberg’s correlation and Rouhani’s void fraction correlation are used together to predict the experimental value of wetted angle accurately. The tube wall is divided into filmwise condensation section and liquid accumulation section by the stratified interface. The local heat transfer coefficient in filmwise condensation section is significantly higher than that in liquid accumulation section. The local heat transfer coefficient increases with vapor quality and steam mass flow rate for all locations along the circumferential direction. The increase of temperature difference between steam and cooling water causes the decrease of local heat transfer coefficient in filmwise condensation section and has less effect on that in liquid accumulation section. With the change of steam saturation temperature, the heat transfer coefficients in two sections of the tube appear different variation tendencies. Based on the experimental values, new correlations for local heat transfer coefficient of steam condensation flow in stratified flow pattern are proposed.

Determination of interface heat transfer coefficient between aluminum casting and graphite mold

To produce castings of titanium, nickel, copper, aluminum, and zinc alloys, graphite molds can be used, which makes it possible to provide a high cooling rate. No die coating and lubricant are required in this case. Computer simulation of casting into graphite molds requires knowledge of the thermal properties of the poured alloy and graphite. In addition, in order to attain adequate simulation results, a series of boundary conditions such as heat transfer coefficients should be determined. The most important ones are the interface heat transfer coefficient between the casting and the mold, the heat transfer coefficient between the mold parts, and the interface heat transfer coefficient into the environment. In this study, the interface heat transfer coefficient h between the cylindrical aluminum (99.99%) casting and the mold made of block graphite of the GMZ (low ash graphite) grade was determined. The mold was produced by milling using a CNC milling machine. The interface heat transfer coefficient was found by minimizing the error function reflecting the difference between the experimental and simulated temperatures in a mold and in a casting during pouring, solidification, and cooling of the casting. The dependences of the interface heat transfer coefficient between aluminum and graphite on the casting surface temperature and time passed from the beginning of pouring are obtained. It is established that, at temperatures of the metal surface contacting with a mold of 1000, 660, 619, and 190°C, the h is 1100, 4700, 700, and 100 W/(m2 K), respectively; i.e., when cooling the melt from 1000°C (pouring temperature) to 660°C (aluminum melting point), the h rises from 1100 to 4700 W/(m2 K), and after forming the metal solid skin on the mold surface and decreasing its temperature, the h decreases. In our opinion, a decrease in the interface heat transfer coefficient at casting surface temperatures lower than 660°C is associated with the air gap formation between the surfaces of the mold and the casting because of the linear shrinkage of the latter. The heat transfer coefficient between mold parts (graphite–graphite) is constant, being 1000 W/(m2 K). The heat transfer coefficient of graphite into air is 12 W/(m2 K) at a mold surface temperature up to 600°C.

DETERMINATION OF HEAT TRANSFER COEFFICIENT BETWEEN ALUMINUM CASTING AND GRAPHITE MOLD

Graphite molds can be used to produce titanium, nickel, copper, aluminum and zinc castings. Using of the graphite molds provides a high cooling rate. Moreover no die coatings and lubricants are required. To get appropriate results of the casting process simulation in graphite molds it is necessary to know thermophysical properties of materials and boundary conditions such as interface heat transfer coefficients, but they are still unknown. The most important properties are heat transfer coefficient between casting and mold, and between mold parts and between mold and environment. The heat transfer coefficient h (iHTC – interface Heat Transfer Coefficient) was determined between cylindrical aluminium (99,99 % Al) casting and mold made of low-ash graphite. The mold was produced by milling graphite blocks on the CNC machine. The heat transfer coefficient was determined by minimizing the error function, representing the difference between the experimental and obtained by simulation temperature in the mold during pouring, solidification and cooling of the casting. The dependences of the iHTC between aluminium and graphite on the temperature of the casting surface and time elapsed from the start of pouring of the casting. Determined values of the heat transfer coefficient at metal temperatures 1000, 660, 619 and 190 °С are 1100, 4700, 700 and 100 W/(m2 ·К) respectively. Therefore, with decreasing of the melt temperature from 1000 °C (pouring temperature) to 660 °C (aluminium melting point), the heat transfer coefficient increases. After casting surface forming and lowering its temperature, the heat transfer coefficient decreases. Decrease of the iHTC at temperatures below 660 °C (lower the melting point) is associated primarily with the appearance of an air gap between the mold surface and casting surface as a result of linear shrinkage. The iHTC between the mold parts (graphite– graphite) is constant 1000 W/(m2 ·К). The heat transfer coefficient between graphite and the air environment is 12 W/(m2 ·К) at the mold surface temperature up to 600 °C.

The lock-in technique applied to heat exchangers: A semi-analytical approach and its application to fouling detection

One of the current methods to detect fouling in heat exchangers is based on the analysis of the evolution of the effectiveness over time. The present results show that, using this kind of analysis, a 6% decrease of the overall heat transfer coefficient cannot be detected when measurement noise (±0.5°C) is taken into account. On the contrary, the present study shows that the analysis of the evolution of the modulus of the variable computed using the lock-in technique is simple and leads to a very sensitive detection when it is well tuned. In this study, the excitation needed for the application of the lock-in technique is a periodical variation of the inlet temperature of one fluid. It is shown that a 4 million sample sliding observation window is necessary to obtain an accurate value of the modulus. This corresponds to a time span from a few hours up to 5days, depending on the sampling period. The latter can then be adapted to the expected fouling rate. One important finding is that, in order to detect fouling, the excitation has to be done on the side where fouling is expected.

Evolution of surface temperature and its relationship with acrylamide formation during conventional and vacuum-combined baking of cookies

In this study, the mechanism by which vacuum baking reduces acrylamide in cookies was elucidated by continuous measurement of average surface temperature during conventional and vacuum-combined baking experiments. Heat transfer coefficient was also determined under atmospheric and vacuum baking conditions. Cookie dough samples (0.3 cm thickness, 5 cm diameter) were baked at 180 °C either completely under atmospheric pressure (101 kPa) or under a combination of atmospheric and reduced pressure (61 and 41 kPa). Baking times were adjusted accordingly to yield cookies with a moisture content between 3.0 and 3.5%. Surface and internal temperatures were recorded throughout baking treatments by using a thermal camera and a thermocouple, respectively. The degree of thermal load (energy input to the product) was then computed from the time - temperature profiles and related to acrylamide level in the finished product by using the total risk calculation approach. Acrylamide concentration of cookies prepared by vacuum-combined baking was found to be around 300 ng/g, a level which is about 30% lower in comparison to that of the conventionally baked sample (445 ng/g). A slight decrease in convective heat transfer coefficient was observed upon pressure reduction. Evaporative cooling effect created by sudden pressure reduction was found to be mainly responsible for limited temperature increase and hence acrylamide mitigation.

Study of peristaltic flow of magnetohydrodynamics Walter's B fluid with slip and heat transfer

The purpose of this paper is to investigate the effects of magnetohydrodynamics peristaltic flow of Walter's B fluid in an inclined asymmetric channel under the influence of slip conditions. The effects of heat and mass transfer are also taken into account. Analytical solutions of nonlinear coupled equations are obtained by regular perturbation method. Graphs for different flow parameters of interest are sketched and analyzed. It is observed that the absolute value of shear stress and heat transfer coefficient decreases by increasing the magnetic parameter whereas with the increase of magnetic parameter, the concentration decreases. Opposite behavior has been noted for temperature and heat transfer coefficient at upper and lower walls against the various values of Prandtl number, Eckert number, slip parameter and material constant of Walter's B fluid. Oscillatory behavior of heat transfer coefficient is observed which is due to propagation of peristaltic waves along the walls of the channel.

Study of turbulent heat transfer of the nanofluids in a cylindrical channel

The experiment investigation of turbulent forced convection of nanofluids with SiO2 and Al2O3 nanoparticles was carried out. Nanoparticle concentration varied in the range from 0.5 to 2vol.% in the experiments. The nanoparticle size ranged from 10 to 100nm. The dependence of heat transfer coefficient and pressure drop from the concentration, size, material of the nanoparticles and temperature was studied. It was shown that adding nanoparticles to the coolant significantly influences the heat transfer coefficient in the turbulent flow regime. It is shown that with increasing nanoparticles concentration, the local and average heat transfer coefficients at a fixed Reynolds number increase. Decrease in heat transfer coefficient with increasing particles concentration may take place at a fixed flow rate. It is shown that, the heat transfer coefficient of the nanofluid in turbulent regime increases with increasing nanoparticles size at a fixed flow rate, while has a certain maximum at a fixed Reynolds number. The effect of nanoparticles material on the heat transfer coefficient and pressure loss has been also demonstrated. It is found that the inlet temperature is another factor having a significant effect on turbulent heat transfer performance of nanofluids.