Deterioration Of Heat Transfer Coefficient Research Articles

Heat transfer characteristics of R‐454B and R‐454B/POE‐oil mixture on smooth and GEWA tube: Alternative to R‐410A

To clarify the compatibility of low GWP refrigerant R-454B alternative to high GWP refrigerant R-410A, the pool boiling heat transfer characteristics is presented subject to lubricant oil POEA-68. The tests were conducted on a smooth and highly structured GEWA-B5H tube at different saturated temperatures of 10, 0, and, −6 °C in 10 to 90 kW/m2 heat flux range. Results show that on both tubes the average heat transfer coefficient (HTC) of pure R-454B at all the saturation temperatures and heat flux is 26 to 34% lower than the pure R-410A. The heat transfer capacity of the GEWA-B5H tube is 2.5 to 5.5 times higher than compared to the smooth copper tube at the same saturation temperature and heat flux for both refrigerant R-454B and R-410A. Yet to elucidate the compatibility of POE oil with R-454B, the low viscosity oil POEA-68 is added by a mass fraction (ω%) of 1% to 10% and 0.25% to 5% for the smooth and GEWA-B5H tube, respectively. The mixing of oil alters the HTC, augmentation or deterioration in HTC compared to pure refrigerant is dependent on the liquid pressure (evaporator pressure), heat flux, boiling surface, and mass fraction of oil.

An ingenious fluidic capacitor for complete suppression of thermal fluctuations in two-phase microchannel heat sinks

Rapid bubble growth within the confinement of microchannels leads to undesired flow oscillations. The flow regime cycles between single-phase, bubbly/slug, backflow, dryout, and rewetting regimes. The large difference between the heat transfer coefficient (HTC) in these regimes induces significant thermal fluctuations (≈ ± 5 ° C) and the overall HTC is low. The intensity of backflow and the time-span of dryout increases with heat flux. As a result, a gradual deterioration in HTC followed by a premature CHF is encountered. Here we introduce an ingenious pulse-dampener for complete mitigation of two-phase thermofluidic fluctuations in microchannel heat sinks. An inflatable latex bladder in the outlet manifold acts as a fluidic capacitor to accommodate the excess vapor. Such on-demand vapor removal weakens backflow to avoid extended dryout. In sharp contrast to the heat sinks without pulse-dampener, a relatively high and monotonically increasing HTC without any thermal fluctuations is observed in experiments with pulse-dampener. A maximum HTC of ≈270 kW/m2. K at a heat flux of ≈2 MW/m2 was observed with a nanostructured microchannel heat sink. The monotonically increasing HTC suggests that the true potential of this pulse-dampener-based heat sink design is much larger than the maximum heat flux of ≈2 MW/m2 tested in our work.

Experimental investigation of natural convection heat transfer characteristics in MWCNT-thermal oil nanofluid

The carbon nanotubes are considered as one of the highest thermal conductive material which is having a variety of heat transfer applications. The suitability of carbon nanotubes in convective heat transfer is examined using multi-wall carbon nanotubes (MWCNT)-thermal oil-based nanofluids. Stable nanofluids are prepared in the concentration range of 0–1 mass% and Prandtl number range of 415 ≤ Pr ≤ 600 using ultrasonication. The natural convection heat transfer behavior is studied experimentally in a vertical rectangular enclosure with aspect ratio 4. The heat transfer experiments are conducted at varying heat flux in the range of 1594–3150 W m−2. The heat transfer coefficient, Nusselt number and Rayleigh number are estimated for MWCNT-thermal oil-based nanofluids and are compared with pure thermal oil. A significant deterioration in heat transfer coefficient is observed at higher concentrations of nanofluids. The study signifies the adverse impact on the cooling performance of MWCNT-thermal oil-based nanofluids in natural convection heat transfer, even though higher thermal conductivities are observed in nanofluids. It is found that not only thermal conductivity is essential property in heat transfer, but other thermophysical properties are also influential towards thermal management.

Numerical Comparison of Thermalhydraulic Aspects of Supercritical Carbon Dioxide and Subcritical Water-Based Natural Circulation Loop

Abstract Application of the supercritical condition in reactor core cooling needs to be properly justified based on the extreme level of parameters involved. Therefore, a numerical study is presented to compare the thermalhydraulic performance of supercritical and single-phase natural circulation loops under low-to-intermediate power levels. Carbon dioxide and water are selected as respective working fluids, operating under an identical set of conditions. Accordingly, a three-dimensional computational model was developed, and solved with an appropriate turbulence model and equations of state. Large asymmetry in velocity and temperature profiles was observed in a single cross section due to local buoyancy effect, which is more prominent for supercritical fluids. Mass flow rate in a supercritical loop increases with power until a maximum is reached, which subsequently corresponds to a rapid deterioration in heat transfer coefficient. That can be identified as the limit of operation for such loops to avoid a high temperature, and therefore, the use of a supercritical loop is suggested only until the appearance of such maxima. Flow-induced heat transfer deterioration can be delayed by increasing system pressure or lowering sink temperature. Bulk temperature level throughout the loop with water as working fluid is higher than supercritical carbon dioxide. This is until the heat transfer deterioration, and hence the use of a single-phase loop is prescribed beyond that limit.

Boiling Heat Transfer of Alumina Nano-Fluids: Role of Nanoparticle Deposition on the Boiling Heat Transfer Coefficient

This paper focuses on the thermal performance of alumina nano-fluids during the quenching process of a surface at the boiling condition, which can be a good answer to the controversial results available in the nano-fluid related literature. For this purpose, an experimental study is conducted to investigate the potential application of alumina/water nano-fluid for cooling a stainless steel rod under the flow boiling heat transfer mechanism. Nano-fluids are prepared by dispersing the 5, 50 and 80nm alumina nanoparticles into the deionized water. The experimental facility provides conditions to quantify the heat transfer coefficient in forced convection and nucleate boiling heat transfer domains at different operating conditions. In terms of operating time, the experiments are divided into two domains namely short time study and extended time study. For the short time study (0-60 minutes of study with neglecting the role of time on the deposition of nanoparticles) enhancement of heat transfer coefficient is reported for all nano-fluids, however for nano-fluid with smaller nanoparticle size, higher thermal performance is registered. In extended time study (60-1000 minutes) heat transfer coefficient is found to be considerably deteriorated for all nano-fluids. This work demonstrates that the reason for deterioration of heat transfer coefficient is referred to the surface roughness, nanoparticle size, static contact angle and thermal fouling resistance parameters. These four parameters are simultaneously determinative factors, which strongly control the thermal behaviour of nano-fluids over the extended time and are the exact reasons for the controversies raised in the literature.

Pool boiling heat transfer to aqueous alumina nano-fluids on the plain and concentric circular micro-structured (CCM) surfaces

The present work mainly focuses on the pool boiling heat transfer of aqueous alumina nano-fluids up to critical heat flux point on a circular surface at two mass concentrations of 0.1%, 0.3% and nanoparticle size of 20nm and 50nm. Nano-fluids were prepared using two-step method and were stabilized by nonylphenolethoxylate (NPE), non-ionic surfactant. Surface of the heater was modified by CNC micro-machinery technique and concentric circular microstructures (CCM) were created on the surface with different geometrical specifications. Results demonstrated that the heat transfer coefficient (HTC), could be deteriorated and enhanced for the plain and micro-structured surface respectively. However, rate of enhancement depended in part, to the particle size and geometrical specifications of microstructures. Rate of enhancement was found to be intensified with increasing the heat flux and mass concentration, decreasing the nanoparticles size and the interspace between microstructures, however height of microstructures had no particular influence on HTC. In terms of critical heat flux point (CHF), nano-fluids expectedly enhanced the CHF value. The enhancement rate of CHF was intensified with increasing the mass concentration of nano-fluid, nano-particle size and interspace between microstructures. The main reason for enhancing the heat transfer coefficient over the micro-structured surface was due to the larger heat transfer area in comparison with plain surface. While for the plain surface, deterioration of heat transfer coefficient was due to the continual fouling formation of nanoparticles on the surface, which increased the static contact angle of nano-fluid on deposited layer and subsequently reduced the number of active nucleation sites. Capillary wicking action and constitution of inflow liquid inside the fouling layer were the main reasons for CHF enhancement since can diminish the surface dry-out regions and enhance the surface toleration against the CHF crisis. Accordingly, a modified mechanism was proposed for enhancing the CHF in nano-fluids, which was the combination of liquid absorption from the bulk of nano-fluids, by the deposition layer and inflow capillary wicking action inside the deposition layer, which leads to the rewetting phenomena in dry-out regions.

Pool boiling heat transfer of water/γ-alumina micro-fluids around the horizontal cylinder

A set of experiments was performed to quantify the pool boiling heat transfer coefficient of water/γ-alumina micro-fluids at mass concentration ranged from 0.1 to 0.4 % of micro-particles with mean size of 1–2 μm. To stabilize the prepared micro-fluid, pH control, stirring and adding the SDS as a surfactant were carried out. Also, thermal conductivity of micro-fluids are measured using KD2 decagon pro. Results showed that micro-fluids have relatively higher thermal conductivity rather than the base fluids. According to the results, there are two distinguishable heat transfer regions namely natural convection and nucleate boiling regions. Influence of some operating parameters such as heat flux, mass concentration of micro-particles and surface fouling resistance on the pool boiling heat transfer coefficient were experimentally studied and briefly discussed. Results demonstrated a significant deterioration of heat transfer coefficient of micro-fluids in comparison with the base fluid over the extended time (1000 min of operation) in nucleate boiling region, while in natural convection region, enhancement of heat transfer coefficient is registered. According to the results, heat transfer coefficient is strongly controlled by/γ-alumina concentration due to the deposition of micro-particles on the heating section. Rectilinear changes of scale formation with time in term of fouling resistance were clearly seen at regions, where natural convection is a dominant heat transfer mechanism and also for higher heat fluxes at nucleate boiling heat transfer region.

Role of nanofluid fouling on thermal performance of a thermosyphon: Are nanofluids reliable working fluid?

This paper focuses on fouling formation of nanofluid in a gravity-assisted thermosyphon equipped with a mesh screen wick. A transient study has been established on the fouling resistance of water-based TiO2 nanofluids at different operating conditions comprising the applied heat flux, mass concentration and inclination of thermosyphon. Nanofluids were prepared using prolonged sonication, stirring, and pH control. Triton X-100 was utilized as a dispersant. The thermosyphon is a copper made tube, which is dimensionally 10.7 mm and 12 mm for inner and outer pipe diameters respectively and total length of 280 mm. Transient results over the extended time reveal a considerable deterioration of heat transfer coefficient and thermal performance as a result of fouling formation inside the wick at evaporator section. According to the results, rate of fouling can be enhanced by increasing the nanofluid mass concentration. Intensification of fouling on wick structure and internal wall of evaporator causes instabilities in thermal performance of thermosyphon over the time, which eventually causes the thermosyphon failure. This point has been ignored by most of previous researchers, which has a considerably negative impact on thermal performance of thermosyphon at longer operating time and higher heat loads. Therefore, a new fouling resistance model has been re-developed combining the microfluidic typical models. This model can predict the fouling resistance of nanofluid inside the thermosyphon with approximate deviation of 30%.

Pool boiling heat transfer to dilute copper oxide aqueous nanofluids

A set of experiments have been performed to quantify the pool boiling heat transfer coefficient of dilute copper oxide water-based nanofluids at mass concentrations of 0.1–0.4%. To stabilize the two-step nanofluids, pH control, stirring and sonication were utilized. For investigating the influence of surfactant as a surface active agent additive on the pool boiling heat transfer coefficient of nanofluids, SDS, SDBS and Triton X-100 were used. Influence of some operating parameters such as applied heat flux, mass concentration of nanofluids and other parameters such as roughness of surface, boiling contact angle and deposition on the pool boiling heat transfer coefficient of nanofluids were experimentally investigated. Results demonstrated a significant deterioration of heat transfer coefficient of nanofluids comparing with the base fluid in the absence of surfactants, however, in the presence of surfactant, higher pool boiling heat transfer coefficient was reported. According to results, roughness of surface is strongly controlled by nanofluid concentration due to deposition of nanofluids on the heating section. Rectilinear changes of deposition with time in term of fouling resistance were seen at both regions with natural convection and nucleate boiling dominant heat transfer mechanisms.

Sedimentation and convective boiling heat transfer of CuO-water/ethylene glycol nanofluids

The convective boiling characteristics of dilute dispersions of CuO nanoparticles in water/ethylene glycol as a base fluid were studied at different operating conditions of (heat fluxes up to 174 kW m−2, mass fluxes range of 353–1,059 kg m−2 s−1 and sub-cooling level of 343, 353 and 363 K) inside the annular duct. The convective boiling heat transfer coefficients of nanofluids in different concentrations (vol%) of nanoparticles (0.5, 1, and 1.5) were also experimentally quantified. Results demonstrated the significant augmentation of heat transfer coefficient inside the region with forced convection dominant mechanism and deterioration of heat transfer coefficient in region with nucleate boiling dominant heat transfer mechanism. Due to the scale formation around the heating section, fouling resistance was also experimentally measured. Experimental data showed that with increasing the heat and mass fluxes, the heat transfer coefficient and fouling resistance dramatically increase and rate of bubble formation clearly increases. Obtained results were then compared to some well-known correlations. Results of these comparisons demonstrated that experimental results represent the good agreement with those of obtained by the correlations. Consequently, Chen correlation is recommended for estimating the convective flow boiling heat transfer coefficient of dilute CuO-water/ethylene glycol based nanofluids.

Thermal evaluation of nanofluids in heat exchangers

Nanofluids, suspensions of nanoparticles (less than 100nm) in a basefluid, have shown enhanced heat transfer characteristics. In this study, thermal performances of nanofluids in industrial type heat exchangers are investigated. Three mass particle concentrations of 2%, 4%, and 6% of silicon dioxide–water (SiO2–water) nanofluids are formulated by dispersing 20nm diameter nanoparticles in distilled water. Experiments are conducted to compare the overall heat transfer coefficient and pressure drop of water vs. nanofluids in laboratory-scale plate and shell-and-tube heat exchangers. Experimental results show both augmentation and deterioration of heat transfer coefficient for nanofluids depending on the flow rate and nanofluid concentration through the heat exchangers. This trend could be explained by the counter effect of the changes in thermo-physical properties of fluids together with the fouling on the contact surfaces in the heat exchangers. The measured pressure drop while using nanofluids show an increase when compared to that of basefluid which could limit the use of nanofluids in industrial applications.

Effect of inclination on the convective boiling performance of a microchannel heat sink using HFE-7100

The effect of inclination on the convective boiling heat transfer characteristics of the dielectric fluid HFE-7100 within a multiport microchannel heat sink having a hydraulic diameter of 825 μm is studied. The inclinations spans from −90° (vertical downward) to 90° (vertical upward), and a flow visualization is also conducted in this study. It is found that the heat transfer coefficient for the vertical upward and horizontal is comparable, and the heat transfer coefficient for 45° upward considerably exceeds other configurations. On the other hand, the enhancement of heat transfer coefficient under inclined arrangement is reduced with the rise of mass flux. The results also indicate that downward arrangements always impair the heat transfer performance, and more than 50% deterioration of heat transfer coefficient is encountered for G = 100 kg m −2 s −1 relative to that of 45° arrangement. The flow visualization confirms that the heat transfer augmentation for upward inclination is due to the rise of slug velocity.

Survey on nucleate pool boiling of nanofluids: the effect of particle size relative to roughness

Pool boiling heat transfer using nanofluids (which are suspensions of nano-sized particles in a base fluid) has been a subject of many investigations and incoherent results have been reported in literature regarding the same. In the past, experiments were conducted in nucleate pool boiling with varying parameters such as particle size, concentration, surface roughness etc. and all sort of results ranging from heat transfer enhancement, deterioration and no effect were reported. This work tries to segregate a survey on pool boiling of nanofluids with respect to particle concentration. This is due to the fact that a major drift in heat transfer behavior is observed at higher and lower particle concentration. But upon deep perusal it has been found that deterioration in heat transfer coefficient are mainly observed at higher particle concentrations (4–16% by weight) and enhancements mainly at lower particle concentrations (0.32–1.25% by weight). Moreover, the relative size of the particle with respect to the surface roughness of the heating surface seems to play an important role in understanding the boiling behaviour. Also, recent works have reported that change in ‘surface wetting’ of the heating surface due to nanofluids and the formation of a porous layer modifiying nucleation site density can be of importance in predicting nucleate pool boiling characteristics of nanofluids. In the present paper, attempts are made to make systematic analysis of results in literature and try to bring out a common understanding of the results in literature.