Maximum Heat Transfer Enhancement Research Articles

Experimental study on heat transfer enhancement using combined surface roughening and macro-structures in a confined double-nozzle spray cooling system

• Spray cooling on enhanced surfaces was experimentally studied. • Combined micro/macro structured surface was used to enhance heat transfer. • An empirical correlation considering surface roughness effect was developed. • A decoupling analysis was performed on the hierarchically enhanced surfaces. • A maximum heat transfer enhancement of 288% was achieved. An experimental study is conducted to characterize the effects of surface roughness and the combined effects of surface roughening and macro-structure topology on the spray cooling heat transfer. It was found that the spray cooling thermal performances increase as the surface roughness increases. From the experimental results, a power law relationship is established between the heat flux and the magnitude of normalized roughness. On this basis, an empirical correlation is developed for spray cooling heat transfer on structured flat surfaces with varying roughness. The correlation is found to have an accuracy of 15%. In addition, the experimental results on the straight finned surfaces are compared with a region-based model with the surface roughness effect incorporated in the developed empirical correlation. The analysis reveals a decoupling relationship between the micro-roughness and macro-structure enhancing mechanisms. Furthermore, a decoupling analysis suggests that the micro-roughness enhancement and the macro-structure enhancement dominate the 0.5 mm pin finned surface and the 1.0 mm pin finned surface, respectively. However, the two types of enhancing mechanisms have a comparable contribution to spray cooling heat transfer enhancement on the straight finned surfaces. In general, spray cooling heat transfer can be enhanced by around 116% by increasing surface micro-roughness whereas heat transfer enhancement can reach as high as 136% and 288% on the surfaces with macro straight fin and pin fin structures containing micro-roughness, respectively.

Experimental Study on Convection Heat Transfer Enhancement of Channel-Flow with Piezoelectric Fan

Experimental work was carried out to investigate the convection heat transfer enhancement of channel flow using a piezoelectric fan oriented along the streamwise direction. The piezoelectric fan was operated at its first natural frequency of 90.3 Hz with various excitation voltages. The average velocity of channel flow was ranged up to 3 m/s, which has the corresponding channel Reynolds number of 3,616. The experimental methodology was numerically and analytically validated. The effects of fan location on the heat transfer performance were evaluated by changing the relative position of the fan tip to the heated surface. The convection heat transfer coefficient and channel pressure drop were measured at different operational conditions of the fan and channel flow. The maximum heat transfer enhancement of 102% was achieved at a flow rate of 15 LPM when the piezoelectric fan was excited with 170 V at the front-end of the heated surface. For a fixed channel flow condition, the piezoelectric fan improved the heat transfer performance only when it was operated with amplitudes larger than the critical value. The overall heat transfer performance of the heated surface in the channel was found to be dependent on the channel flow rate and the amplitude and frequency of the piezoelectric fan.

Enhancement of heat transfer of [formula omitted] based nanofluid flow through 45° angled slot ribbed square duct

In the present work, the response of 45o angled slot ribbed square duct when conventional working fluid (water) is replaced by nanofluids is tested. The square conduit top angled slot ribbed wall heated to a consistent temperature. Silica type of nanoparticle is used, with a constant volume fraction of 3.5 percent and a nanoparticle diameter of 35 nm dispersed in water as the base fluid. Various parameters such as Reynolds number in the range of 3000 < Re < 9000, slot rib height 0.05 < Hrib/Dh < 0.13 and other slot rib parameters such as stream wise pitch (Xaxis/Dh) of 0.394 and span wise pitch (Yaxis/Dh) of 0.22 were fixed in the current study. Maximum heat transfer enhancement is achieved with an angled 45o slot rib height of 0.08, which is two times higher than the plain wall.

Augmentation of Heat Transfer in a Circular Channel with Inline and Staggered Baffles

This numerical investigation presents the effects of the position of baffles in the shape of a circle’s segment placed inside a circular channel to improve the thermal and flow performance of a solar air heater. Three different baffles’ positions with Reynolds number varying between 10,000 to 50,000 were investigated computationally. The k-omega SST model was used for solving the governing equations. Air was taken as the working fluid. Three pitch ratios (Y = 3, 4, and 5) were considered, while the height of the baffles remained fixed. The result showed an enhancement in Nusselt number, friction factor, j-factor, and thermal performance factor. Staggered exit-length baffles showed maximum enhancement in heat transfer and pressure drop, while inline inlet-length baffles showed the least enhancement. For a pitch ratio of Y = 3.0, the enhancement in all parameters was the highest, while for Y = 5.0, the enhancement in all parameters was the least. The highest thermal performance factor of 1.6 was found for SEL at Y = 3.0.

Effect of magnetic field on heat transfer from a channel: Nanofluid flow and porous layer arrangement

Due to the high number of porous media applications in industries, the demands for analyzing the porous medium's flow and heat transfer are rising every day. The present research intends to evaluate the impact of porous media, nanofluid, and magnetic field on heat transfer of a circular channel. Two typical porous arrangements are considered: central configuration and boundary configuration. It is of interest to know the impact of porosity, thickness, permeability, and thermal conductivity ratio in porous media. The working fluid, nanoparticles and porous medium are water, CuO and steel foam, respectively. The results reveal that the heat transfer rate in the central arrangement is more than the boundary arrangement. When the non-dimensional thickness of the porous media is 0.8 in the central arrangement, the heat transfer rate is at its peak. Simultaneously, the minimum happens when the non-dimensional thickness is set to 0.6 in the boundary arrangement. Applying nanofluid and increasing the volume fraction will improve the heat transfer rate. The average heat transfer coefficient is increased when the magnetic field is applied up to the intensity of 0.5 T. Additionally, the maximum heat transfer enhancement is achieved when the thickness is 0.6 in boundary arrangement in the case of applying the magnetic field, which is estimated to be 3–5% more. Modifying the shape of the porous media in the boundary arrangement decreases the heat transfer rate about 7–21%, depending on the shape compared to a homogeneous boundary porous.

Investigation of enhancement in the thermal response of phase change materials through nano powders

A major problem faced by Phase Change Materials (PCM) based latent heat thermal energy storage systems is the slow thermal response. In this study, the feasibility of nano phase change materials composites is investigated experimentally by using various types of nanoparticles in PCM, to improve thermal response by increasing the thermal conductivity. Heat transfer enhancement of PCM composites (RT26 and coconut oil) is achieved by acquiring better thermophysical properties through seeding nanoparticles in PCM. RT26 and coconut oil are beneficial due to the absence of corrosion to metallic containers. A thermal response investigation is conducted with a flat slab type latent heat thermal energy storage unit. It was observed that the increase in thermal conductivity of PCM by seeding nanoparticles is useful for the improvement of heat transfer. Carbon nanotubes have shown better performance as compared to Al2O3 and Fe3O4 nanoparticles. It was further analyzed that at a 1 wt percent concentration of nanoparticles, the maximum heat transfer enhancement in RT26 caused by Fe3O4, Al2O3 and carbon nanotubes nanocomposites is up to 20.01, 36.68 and 64.21% respectively. The maximum heat transfer enhancement in coconut oil caused by Fe3O4, Al2O3 and carbon nanotubes nanocomposites is up to 8.83, 14.84 and 33.38% respectively. Therefore, it is revealed that RT26 has more potential for heat transfer enhancement as compared to coconut oil. This is due to the percentage improvement in the thermophysical response of RT26 as compared to coconut oil. The economic and environmental analysis was conducted to compare the performance of latent heat thermal energy storage unit with and without the application of nanoparticles in PCM, to estimate the most viable candidate among various nano composites. The economic analysis presented the lowest annual energy running cost and Net Present Value(NPV) for nanocomposite of carbon nanotubes in RT26 is Rs.-4381 and RS.-36375 respectively. Whereas the maximum annual energy cost and NPV for pure RT26 are Rs.-12240 and Rs. -75548 respectively. The NPV for Fe3O4 and Al2O3 is Rs.-62207 and Rs.-53062 respectively. The cost and NPV were Rs.-6646 and Rs.-55226 in the case of carbon nanotubes in coconut oil. It shows that the trend is the same in the case of Fe3O4 and Al2O3 nanoparticles in both RT26 and coconut oil, unlike of carbon nanotubes. The environmental analysis shows that the maximum and minimum payments of carbon dioxide by carbon nanotubes and Fe3O4 nano composites respectively in RT26, are Rs.7000 and Rs.2176. While the corresponding values are Rs.2975 and Rs.798 in the case of coconut oil. Hence, carbon nanotubes in RT26 have the highest economic and environmental viability. Accordingly, the enhancement in heat transfer using these nanoparticles will be helpful in air conditioning applications like animal houses.

Heat transfer and flow visualization of pulsating heat pipe with silica nanofluid: An experimental study

As heat transfer device with two-phase flow, pulsating heat pipe (PHP) has a widely application prospect in the fields of improving energy efficiency and thermal management. In order to optimize the heat transfer performance of PHP, a visualization experiment was carried out with a high-speed camera, and detailed characteristics of flow pattern variations in nanofluid PHP were obtained. The effect of SiO2-H2O nanofluid with different concentrations (0.5 wt%, 1.0 wt%, 1.5 wt% and 2.0 wt%) were experimentally investigated to analyze the thermal performance of PHP. Results demonstrate that the addition of nanoparticles on the one hand promotes the phase transition of the working fluid in the PHP, and on the other hand increases the instantaneous velocity and the driving force of working fluid. These are all benefit for the reflux of condensed liquid and effectively avoid the phenomenon of ′dry out′. Additionally, the results also indicate that there exists an optimum concentration of dispersed nanoparticles in base fluid to enhance heat transfer. The maximum heat transfer enhancement efficiency with nanofluid concentration of 1.0 wt% can reach 40.1 % at the heating power of 50 W. The present results can provide useful inspiration for the development and application of new type pulsating heat pipe.

A comparative analysis on the single-phase and two-phase mixture models for calculation of ferrofluid convective heat transfer in the presence of a magnetic field: a numerical study

PurposeThis paper aims to analyze the accuracy of the single and two-phase numerical methods for calculation of ferrofluid convective heat transfer in the presence of a magnetic field. The findings of current study are compared with previous single-phase numerical results and experimental data. Accordingly, the effect of various parameters including nanoparticles concentration, Reynolds number and magnetic field strength on the performance of the single and two-phase models are evaluated.Design/methodology/approachA two-phase mixture numerical study is carried out to investigate the influence of four U-shaped electromagnets on the hydrodynamic and thermal characteristics of Fe3O4/Water ferrofluid flowing inside a heated channel.FindingsIt is observed that the applied external magnetic field signifies the convective heat transfer from the channel surface, despite local reduction at a few locations. The maximum heat transfer enhancement is predicted as 23% and 25% using single and two-phase models, respectively. The difference between the results of the two models is mainly attributed to the slip velocity effect which is accounted for in the two-phase model. The magnetic field gradient leads to a significant increase in the slip velocity which in turn causes a slight difference in velocity and temperature profiles obtained by the single and two-phase models in the magnetic field region. According to percentage error calculation, the two-phase method is generally more accurate than the single-phase method. However, the percentage error of both models improves by decreasing either magnetic field intensity or Reynolds number.Originality/valueFor the first time in the literature, to the best of the authors’ knowledge, the current work analyzes the accuracy of the single and two phase numerical methods for calculation of ferrofluid convective heat transfer in the presence of a magnetic field.

Comparative analysis of microfin vs smooth tubes in R32 and R410A boiling

• This paper investigates R32 and R410A boiling inside a 4.2 mm microfin tube. • The microfin tube exhibits a maximum heat transfer enhancement around 1.8 − 1.9. • In general the heat transfer enhancement is lower than the mere surface area increase. • R32 exhibits heat transfer coefficients and pressure drops higher than R410A. • R32 is a very promising low GWP substitute for R410A. This study performs the comparative analysis of R32 and R410A boiling inside a 4.2 mm ID microfin tube. The experimental tests were carried out at different saturation temperatures, different vapour qualities, heat fluxes and mass velocities to evaluate the specific contribution of each operating parameter. The boiling heat transfer coefficients show a great sensitivity to heat flux and vapour quality and no sensitivity to refrigerant mass velocity. The microfin tube exhibits a maximum heat transfer enhancement with respect to the smooth tube around 1.8 − 1.9 at a heat flux of 12 kW m −2 and a vapour quality of 0.8, and in general the heat transfer enhancement is lower than the mere surface area increase over the smooth tube (Rx = 1.65). R32 exhibits boiling heat transfer coefficients and frictional pressure drops 10 − 20% higher than those of R410A and it seems a valuable low GWP substitute for R410A.

An insight into the performance of radiator system using ethylene glycol-water based graphene oxide nanofluids

Recent studies showed that nanofluids are considered as a promising way to enhance the heat transfer properties of coolants. Currently, in a car radiator forced convection heat transfer method is carried out to cool circulating fluid. The mixture of water with either ethylene or propylene-glycol is necessary to lower its freezing point and eliminate ice formation. However, the boiling point of deionized water can be pushed up by mixing it with anti-freezing materials like ethylene glycol (EG). Hence the present research work focuses on the heat transfer characteristics of nano-fluids with different combinations of EG and deionized water (60:40, 30:70, and 20:80) along with graphene oxide (GO) nanoparticles (0.1 % wt). The heat transfer potential of graphene oxide/ deionized water-ethylene glycol nanofluids is experimentally investigated in coolant application for car radiators, in which the coolant flow rate is varied from 180 to 420 (LPH) at a fixed coolant temperature of 90 °C. The evaluation of stability is performed using visual inspection and zeta potential test. The density of nanofluid was determined by oscillating U-Tube density meter. The optimized combination of 60% EG, 40% DW and 0.1 wt% GO exhibited higher heat transfer characteristics of the radiator system used. The maximum heat transfer enhancement of 42.77% at 300 LPH, 18.14% at 360 LPH, and 71.1% at 240 LPH for 60:40 EG/DW-based GO nanofluids were obtained. The maximum Nusselt number value of 192 at 420 LPH was observed for 60:40 EG/DW-based GO nanofluid as compared with 20:80, 30:70 EG/DW-based GO nanofluid. It is estimated that by replacing conventional coolant with this nanofluid, reduction in the frontal area of radiator is done which gives more flexibility for industrial design, more eco-friendly vehicle which produces less drag and hence less the fuel cost.

Numerical study of heat transfer and flow structure over channel surfaces featuring miniature V rib-dimples with various configurations

In order to enhance the heat transfer performance of the internal cooling for gas turbine blades, miniature V rib-dimple hybrid structures on cooling channel surfaces were studied numerically at Re = 50,500, and the effects of parameters of rib heights, dimple depths, rib pitches and rib-to-dimple pitches were detailly investigated on the turbulent flow structure, pressure loss and heat transfer characteristics. The Abe Kondoh Nagano (AKN) k−ε turbulence model was utilized for the numerical simulation work. The results showed that as the rib height and dimple depth increase, the globally averaged heat transfer augmentation increases significantly at first, and then becomes stable. And the maximum heat transfer enhancement is reached when the rib height-to-channel hydraulic diameter ratio is 0.075 and the dimple depth-to-diameter ratio is 0.2. Besides, the more densely fabricated hybrid structures induce higher heat transfer, while a relatively larger spacing between the rib and the downstream dimple also obtains higher heat transfer. Within the parameters of this study, the total heat transfer enhancement of the hybrid structures can be up to 95.3% and 43.2% respectively higher than that of the dimple-only and the rib-only structures. The optimal hybrid structure with the overall thermal performance factor of 1.73, with e/Dh=0.075, δ/d = 0.2, P1/e = 7.2 and L/P1 = 0.83, has a total Nusselt number ratio of 3.9 and a friction ratio of 11.6. It is evident that with the miniature V rib-dimple hybrid structure, the turbulent kinetic energy of the flow passing over the V ribs is enhanced, thereby reducing the recirculation zone in the front of the dimples. Additionally, the vortex flow induced by the V rib strongly interacts with the downstream dimpled wall which significantly augments the overall momentum and heat transport of the near-wall flow field, and contributes to enhanced and more uniformly distributed heat transfer on the channel surface.

Forced convection heat transfer of aviation kerosene enhanced by electric field in a circular channel

• Using EHD technique to enhance avionics system heat dissipation is proposed. • Electrical properties of aviation kerosene in wire-tube EHD structure are studied. • Inlet parameters affecting the EHD heat transfer enhancement is investigated. • Flow, temperature and electric fields are captured by numerical simulation. Coolant heat transfer capacity determines the security of avionics system operation. Electrohydrodynamics (EHD), as an enhanced heat transfer technology, can be applied to the avionics cooling system. In this paper, the forced convection heat transfer characteristics of aviation kerosene enhanced by the EHD technique were investigated by both experimental and numerical simulation methods for the first time. One kind of wire-tube structure heat exchanger was designed and the heat dissipation performance of aviation kerosene was tested experimentally at different inlet parameters and electric potentials. A 3D mathematical model was established based on Maxwell and Navier-Stokes equations and the finite volume method was used for conservative discretization of the governing equations. The experimental and numerical simulation results are in good agreement. In the range from 128 to 900 of Reynolds number, the EHD heat transfer enhancement effect becomes weaken with the increase of mass flow. The charge migration along the electric field is suppressed due to the increasing axial velocity. However, the heat transfer enhancement effect is more obvious with the increasing inlet temperature of aviation kerosene. Due to the low viscosity and high ion mobility of high-temperature aviation kerosene, the charge migrating ability is enhanced and the viscous resistance is easier to overcome to produce the secondary flow. The maximum heat transfer enhancement ratio can reach 1.9 times. This paper expanded the EHD application to aviation fuels as the coolant in avionics systems and provided a deeper understanding on inlet parameters affecting the EHD heat transfer enhancement.

Heat transfer enhancement in mini channel heat sinks utilizing corona wind: A numerical study

A full-scale three-dimensional model of a rectangular channel mini heat sink is developed to numerically investigate the feasibility of heat transfer enhancement utilizing an external electric field. Heat transfer mechanism in the presence of corona wind induced by a wire electrode and its favorable impact on the hydrodynamic and thermal characteristics of working fluid coolant are elaborately discussed. Effects of various parameters such as number of electrodes, electrode radius, electrode location, and electrode-flow configuration as well as the external applied voltage and Reynolds number on heat transfer enhancement are investigated. This study indicates that the electric field creates a vortex which in turn causes flow mixing in the vicinity of the heated surface, disturbs the thermal boundary layer and consequently increases the heat transfer rate. Results show that heat transfer enhancement is directly and inversely proportional to the voltage and Reynolds number, respectively. Furthermore, it is indicated that increasing the number of electrodes leads to a significant reduction in the heat transfer rate unfavorably impacting the heat transfer enhancement. It is attributed to the simultaneous interaction between the adjacent induced vortices. Eventually, it is shown that perpendicular configuration of the flow-electrode layout performs better than its parallel counterpart where an induced lateral air flow reduces the heat transfer augmentation. A maximum heat transfer enhancement of 439% compared to that of without any electrical field case is achieved by using a single electrode in the perpendicular electrode-flow layout.

Nanoparticles to augment Melting process in dynamicall adjusted Thermal Energy Storage Systems: An Analytical Investigation

In the present study, scaling theory is applied to quantitively analyze the potential of nanoparticles to augment melting processes inside constant thermal capacity and constant volume Thermal Energy Storage (TES) systems. The dimensions of constant thermal capacity TES systems are dynamically adjusted, upon the addition of nanoparticles, to maintain constant thermal capacity. The volume fraction of nanoparticles is optimized to achieve the maximum enhancement in heat transfer, and analytical correlations are developed to estimate the effect of optimal volume fraction of nanoparticles on melting time and mean heat transfer rate. The threshold of nanoparticles to augment melting processes, i.e., maximum enhancement in mean heat transfer and reduction in melting time, is calculated. A critical thermal conductivity ratio of nanoparticles to PCM is identified. Furthermore, numerical analyses are conducted, and the results obtained through numerical analysis show good agreement with analytical results. It is observed that the nanoparticles have the potential to enhance the mean heat transfer rate by up to 67.5%.

Experimental investigation on the convection heat transfer enhancement for heated cylinder using pulsated flow

The paper presents experimental results regarding heat transfer behavior of a cylinder in cross pulsating flow and focuses on heat transfer characteristics enhancement obtained using a novel and peculiar type of pulsation technique able to generate variable pulsate signals with different frequencies. The pulsating flow tunnel features a square cross section of 200 mm × 200 mm and 1600 mm length, while the testing apparatus utilizes a station of normally closed (NC) solenoids and a vacuumed plenum chamber which can generate a time dependent flow inside the tunnel. Effects of pulsating frequency in the range 1–12 Hz and Reynolds number in the range 1.02–2.04*104 on convective heat transfer for both a horizontal and vertical heated cylinder are carried out. A cylindrical cartridge heater is embedded into the cylinder with two insulated ends to generate a constant heat flux of 33 W in all the experimental runs. The surface temperature of the cylinder was measured using an array of k-type thermocouples around the cylinder circumference and recorded in a real-time data logger. The results show that the measured velocity and convection heat transfer coefficient are very sensitive to the frequency of crossflow pulsation. In addition, the influence of pulsating frequency on the local and average of heat transfer coefficient is discussed in detail. Results also demonstrate that maximum enhancement in heat transfer for the horizontal and vertical setup occurs at a frequency of 9 Hz compared to steady forced convection, with 8.33% and 11.11% increment in heat transfer respectively.

Comparison of metal oxide and composite phase change material based nanofluids as coolants in mini channel heat sink

Present work focuses on heat transfer and pressure drop studies of Al2O3 based nanofluids and nanoencapsulated composite phase change material (NECPCM) based nanofluids in mini channel heat sink. Composite PCM was prepared by homogeneously mixing paraffin wax and petroleum jelly. The encapsulation of composite PCM in polystyrene shells was done by mini emulsion polymerization and various characterizations were done to study phase change behaviour. The concentration of NECPCM and Al2O3 nanofluids was fixed based on zeta potential measurements. Experiments were conducted in mini channel integrated with a heat sink, consisting of 13 rectangular channels and hydraulic diameter of 2.6 mm. Maximum heat transfer enhancement of 28% was obtained for NECPCM nanofluid against 10% enhancement for Al2O3 nanofluid. Higher pressure drop was observed for Al2O3 nanofluid as compared to NECPCM nanofluids, especially at higher flow rates. Maximum improvement in figure of merit (FOM) obtained for NECPCM nanofluid and Al2O3 nanofluid was 24% and 5% respectively, as compared to DI water. The experimental results show that there exists an optimum flow rate for NECPCM based nanofluids to achieve maximum FOM, which depends on length of the channel and residence time of particles.

Role of fluid-structure interaction in free convection in square open cavity with double flexible oscillating fins

The problem of transient free convection in an open cavity containing two elastic thin fins is studied in this paper. The left surface is heated, and the right surface is in opening, which is in contact with the environment air. The dimensionless forms of the governing equations for continuity, momentum and heat transfer in the fluid and structures domains are written in Arbitrary Lagrangian–Eulerian (ALE) formulation. The governing equations are solved by the finite element method (FEM). The governing parameters considered are the oscillating fin direction, fin amplitude and length with several flexibility values and aperture sizes. The results show that a maximum heat transfer enhancement occurs at the suitable parameters between the aperture size and the fins oscillating direction.

Effect of liquid cooling on PCR performance with the parametric study of cross-section shapes of microchannels

In recent years, PCR-based methods as a rapid and high accurate technique in the industry and medical fields have been expanded rapidly. Where we are faced with the COVID-19 pandemic, the necessity of a rapid diagnosis has felt more than ever. In the current interdisciplinary study, we have proposed, developed, and characterized a state-of-the-art liquid cooling design to accelerate the PCR procedure. A numerical simulation approach is utilized to evaluate 15 different cross-sections of the microchannel heat sink and select the best shape to achieve this goal. Also, crucial heat sink parameters are characterized, e.g., heat transfer coefficient, pressure drop, performance evaluation criteria, and fluid flow. The achieved result showed that the circular cross-section is the most efficient shape for the microchannel heat sink, which has a maximum heat transfer enhancement of 25% compared to the square shape at the Reynolds number of 1150. In the next phase of the study, the circular cross-section microchannel is located below the PCR device to evaluate the cooling rate of the PCR. Also, the results demonstrate that it takes 16.5 s to cool saliva samples in the PCR well, which saves up to 157.5 s for the whole amplification procedure compared to the conventional air fans. Another advantage of using the microchannel heat sink is that it takes up a little space compared to other common cooling methods.

Comparative analysis of microfin vs smooth tubes in R32 and R410A condensation

This study performs the comparative analysis of R32 and R410A condensation inside a 4.2 mm ID microfin tube: the effects of saturation temperature, vapour quality and mass velocities are evaluated. The heat transfer coefficients show a great sensitivity to vapour quality, a non-negligible sensitivity to saturation temperature/pressure, and a non-monotonic trend versus mass velocity. The microfin tube exhibits a maximum heat transfer enhancement with respect to the smooth tube around 3.5−3.6 at a mass velocity of 200 kg m−2s−1 and a vapour quality of 0.8. The optimum mass velocity for R32 and R410A condensation inside the microfin tube is in the range 150−300 kg m−2s−1. R32 exhibits condensation heat transfer coefficients 20 − 30% higher and frictional pressure drops 10 − 20% higher than those of R410A and it seems a valuable low GWP substitute for R410A.

Heat Transfer and Hydrodynamic Properties Using Different Metal-Oxide Nanostructures in Horizontal Concentric Annular Tube: An Optimization Study.

Numerical studies were performed to estimate the heat transfer and hydrodynamic properties of a forced convection turbulent flow using three-dimensional horizontal concentric annuli. This paper applied the standard k–ε turbulence model for the flow range 1 × 104 ≤ Re ≥ 24 × 103. A wide range of parameters like different nanomaterials (Al2O3, CuO, SiO2 and ZnO), different particle nanoshapes (spherical, cylindrical, blades, platelets and bricks), different heat flux ratio (HFR) (0, 0.5, 1 and 2) and different aspect ratios (AR) (1.5, 2, 2.5 and 3) were examined. Also, the effect of inner cylinder rotation was discussed. An experiment was conducted out using a field-emission scanning electron microscope (FE-SEM) to characterize metallic oxides in spherical morphologies. Nano-platelet particles showed the best enhancements in heat transfer properties, followed by nano-cylinders, nano-bricks, nano-blades, and nano-spheres. The maximum heat transfer enhancement was found in SiO2, followed by ZnO, CuO, and Al2O3, in that order. Meanwhile, the effect of the HFR parameter was insignificant. At Re = 24,000, the inner wall rotation enhanced the heat transfer about 47.94%, 43.03%, 42.06% and 39.79% for SiO2, ZnO, CuO and Al2O3, respectively. Moreover, the AR of 2.5 presented the higher heat transfer improvement followed by 3, 2, and 1.5.