Forced Convection Heat Transfer Coefficient Research Articles

Correlations for forced convective heat transfer coefficients at the windward building façade with vertical louvers

Vertical louvers are a widely used external shading system to control natural lighting and solar heat gain for buildings. However, when installed on a building façade, they can disrupt the flow field around the façade and affect the external convective heat transfer process of building's exterior surfaces. To this end, this paper conducted CFD simulations to clarify the forced convective heat transfer patterns in a vertical louver-facade system based on the k-ω SST model with a detailed verification. The extended Newton's cooling equation is used to describe the heat exchange process between louvers, building façade, and air, which generates four heat transfer coefficients: hwa, hwb, hba, and hbw. Then the impacts of dimensionless temperature difference (θ), louver angle (φ), louver slat width (B), and installation distance (W) on the heat transfer coefficients are investigated. The results show that θ does not affect the average heat transfer coefficients. hwa reaches a minimum value and hba reaches a maximum value when φ = 75o. In addition, hwa increases by 55.3% and hba decreases by 21.9% as B increases from 0.3 m to 1 m. When W increases from 0.25 m to 1 m, hwa decreases by 5.6% and hba increases by 39.6%. The variation in the heat transfer coefficients (hwb and hbw) between the louvers and building façade is also explained. Finally, new correlations of paired heat transfer coefficients are established as a function of φ, B, W, and wind velocity (U10). The accuracy of these correlations is evaluated by comparing calculated values with simulated results.

Study on refined mathematical model of solar chimney power plant integrated with seawater desalination and the influence of dewing

In order to more accurately evaluate the operation and output characteristics of the solar chimney power plant integrated seawater desalination, a refined mathematical model for unsteady heat and mass transfer was built under consideration of the convection heat transfer mechanism of the collector, dewing phenomenon, and the disturbance to the hot airflow caused by the oblique-toothed flow channel boundary. The influence of nighttime dewing and humid environments on system performance was discussed. The results show that it is feasible and more appropriate to use the forced convection heat transfer coefficient of bellows to consider the influence of the oblique-toothed flow channel boundary; the refined mathematical model is also more accurate and reliable. Due to the effect of seawater heat storage, dewing has little effect on system performance, and the integrated system has a better anti-dewing negative impact characteristic. However, when the ambient humidity reached a higher level, dewing had a greater negative impact on freshwater production. The critical value of relative humidity for dewing to occur is 51 %. When the relative humidity increases from 45 % to 60 %, the daily utilization efficiency of solar energy decreases from 27.4 % to 22.5 %.

IMPROVING THE THERMAL PERFORMANCE OF A CONCENTRIC-TUBE HEAT EXCHANGER SYSTEM UTILIZING A NOVEL HYBRID MAGNETIC NANOFLUID

Hybrid magnetic nanofluid is the phase involved in a suspension of a mixture of nanometer-sized particles in traditional fluids. The most conspicuous attributes of this fluid comprise improved heat characteristics, for instance, convective heat transfer coefficient, compared to the conventional fluid. Hybrid magnetic nanofluid of iron oxide and ferric oxide with a mixture proportion of 50:50 was added to distilled water (DW). The impact of forced convective heat transfer coefficient in turbulent flow was estimated by employing parallel- and counter-flow concentric-tube heat exchanger systems. The forced convective heat transfer coefficient of the hybrid magnetic nanofluids was calculated applying empirical equations corresponding to the experimental results. Furthermore, the system's performance with (Fe<sub>3</sub>O<sub>4</sub>@Fe<sub>2</sub>O<sub>3</sub>/DW) 0.5 wt.% hybrid magnetic nanofluid with (Fe<sub>3</sub>O<sub>4</sub>/DW), (Fe<sub>2</sub>O<sub>3</sub>/DW) 0.3 wt.% regular magnetic nanofluids, and distilled water was compromised. The determinations reveal notable improvement in the convective heat transfer coefficient in both parallel- and counter-flow regimes in the case of hybrid magnetic nanofluid compared to the regular ones and base fluid, the highest enhancement in the overall convective heat transfer coefficient was up to 49.8% compared to distilled water at 31,689 Reynolds number and 18.46% and 20.5% compared to Fe<sub>3</sub>O<sub>4</sub>/DW and Fe<sub>2</sub>O<sub>3</sub>/DW regular magnetic nanofluids at 36,215 Reynolds number in the parallel-flow regime. Moreover, the Nu number was improved in the case of the hybrid magnetic nanofluid reaching maximum values up to 38.1%, 13.2%, and 14.8% corresponding to distilled water and regular magnetic nanofluids, respectively. The augmentation in the heat transfer utilizing hybrid magnetic nanofluids was caused by 22.47% thermal conductivity improvement compared to base fluid.

CFD analysis of a flat tube with semi‐circular fins using graphene nanofluid and to compare performance with square type of fins

AbstractCFD analysis on a flat tube with semi‐circular fins under laminar flow conditions was performed with graphene‐based nanofluids considering the nanofluids as incompressible. Different simulations were performed at four different concentrations of nanofluids (0.01%, 0.1%, 0.2%, and 0.4%) and at different volume flow rates (4, 6, 8, and 10 LPM) and at four different forced convective heat transfer coefficients at different wind velocities at 300 K (50, 100, 150, and 200 W/m2 K). It was observed that with an increase in the concentration of nanoparticles in nanofluids, the thermal conductivity of base fluid water was increased (at 353 K the nanofluid of 0.4% volume concentration, the thermal conductivity of nanofluid increased by 200% with respect to base fluid). Graphene‐based nanofluids have higher effectiveness than most nanofluids hence it is considered for the analysis, at 0.4% concentration of nanofluid the effectiveness observed was 36.84% at 4 LPM, and for water, the effectiveness was 28.22% under similar conditions. The effect of flow rate on temperature drop was significant. At 4 LPM and at 0.4% of nanofluid, an outlet coolant temperature of 333 K was observed whereas the water outlet temperature at 10 LPM is 346.13 K. The effect of forced convective air heat transfer coefficient was significantly high. At h = 50 W/m2 K the outlet temperature of 0.4% nanofluid at 4 LPM was 345.25 K and at h = 200 W/m2 K, the outlet coolant temperature was 333.47 K. A single tube of the radiator was considered for the analysis whereas the original radiator consists of 50 tubes due to problems of Ansys in meshing.

An experimental study on the heat transfer characteristics over a radiant cooled wall exposed to mixed and forced convection driven by displacement ventilation

An increasing demand on energy efficiency and indoor air quality exists, therefore displacement ventilation and radiant surface cooling applications are taking more attention as an investigation subject. This article presents an experimental study of mixed convection in a test chamber equipped with both radiant wall cooling and air-conditioning systems. The room is conditioned by a hydronic circuit with embedded pipes in the wall and an air jet supplied by a newly designed baseboard longitudinal slot that is apart from relevant literature. The tested radiant wall temperature is kept around 14 °C. Meantime; a parallel upward directional air jet is applied over the cooled wall or the opposite wall. The air jet velocity and its temperature lie within the intervals of 0.25–10 m/s and 18–28 °C, respectively. The natural, mixed, and forced convection heat transfer characteristics over the wall have been investigated considering the effects of radiant cooling, airflow rates, and ventilation location. The measured data are used for the derivation of mixed and forced average convective heat transfer coefficients correlations along a cooled wall. Consequently, the convection and total heat transfer coefficients over the radiant cooled wall are calculated in the ranges of 2–7.3 W/m2K, and 9.1–13.9 W/m2K, respectively.

Forced Convective Heat Transfer Coefficient Measurement of Low Concentration Nanorods ZnO-Ethylene Glycol Nanofluids in Laminar Flow.

This paper presents the experimental forced convective heat transfer coefficient (HTC) of nanorods (NRs) zinc oxide–ethylene glycol nanofluids (ZnO–EG NFs) in laminar flow. First, ZnO NRs were synthesized using a hydrothermal method that uses zinc acetate dihydrate [Zn(CH3COO)2·2H2O] as a precursor, sodium hydroxide as a reducing agent, and polyvinylpyrrolidone (PVP) as a surfactant. The hydrothermal reaction was performed at 170 °C for 6 h in a Teflon-lined stainless-steel tube autoclave. The sample’s X-ray diffraction (XRD) pattern confirmed the formation of the hexagonal wurtzite phase of ZnO, and transmission electron microscopy (TEM) analysis revealed the NRs of the products with an average aspect ratio (length/diameter) of 2.25. Then, 0.1, 0.2, and 0.3 vol% of ZnO–EG NFs were prepared by adding the required ZnO NRs to 100 mL of EG. After that, time-lapse sedimentation observation, zeta potential (ζ), and ultraviolet-visible (UV–vis) spectroscopy was used to assess the stability of the NFs. Furthermore, the viscosity (μ) and density (ρ) of NFs were measured experimentally as a function of vol% from ambient temperature to 60 °C. Finally, the HTC of NFs was evaluated utilizing a vertical shell and tube heat transfer apparatus and a computer-based data recorder to quantify the forced convective HTC of NFs in laminar flow at Reynolds numbers (Re) of 400, 500, and 600. The obtained results indicate that adding only small amounts of ZnO NRs to EG can significantly increase the HTC, encouraging industrial and other heat management applications.

Thermo-fluidic analysis of a single piezofan in longitudinal channel

Most past surveys and conclusions have clearly indicated that more than 55% of electronic device's failures are related to the inability to control high heat generated entirely. The conventional cooling methods typically consist of air-cooling processes, which involve various types of heat sinks in free and forced convection, becomes inadequate to meet the cooling requirements with high performance, compact size, and increased heat generation advanced electronic devices.The main aims of this investigation are (i) to analyze the airflow around a horizontally oriented piezofan, which is oscillating transversally to longitudinal channel airflow, and (ii) to show its applicability to achieve successful thermal management on a flat heated surface at an electronic component. The coupled-analysis was performed on a 2D-laminar flow model by combining the cantilever motion analysis with fluid flow and heat transfer using COMSOL Multiphysics software. The model based on the finite element method (FEM) is determined using an Arbitrarily Lagrange-Euler scheme (ALE) in Fluid-Structure-Interaction (FSI) analysis. The cooling performance of piezofan is computed by evaluating the average forced convective heat transfer coefficient at the heated section of the channel surface with an inlet flow velocity of uin = 0.1 m/s, i.e., Rem = 1040. The simulation results have shown the high performance of these fans compared to without piezofan flow conditions. The best performance was obtained at which the moving tip of the fan at the middle of the heated surface (∆xp = −20 mm) with a distance from the heated surface of G = 7 mm. Compared with the laminar flow conditions without piezofan, the convection heat transfer coefficient is increased by 231%.

메탈 하이브리드 소재를 활용한 전기자동차 모터의 스마트 설계 및 냉각 성능 향상에 대한 연구

The aim of this study is to numerically investigate the cooling performance of the electric vehicle motor depending on the attachment of the heat sink and materials to the cooling channel. The research focused on the numerical comparison of forced convective heat transfer coefficients with case 1 (Heat Sink-None, Cooling Channel-Al), case 2 (Heat Sink-None, Cooling Channel-Metal Hybrid Material), case 3 (Heat Sink-4EA, Cooling Channel-Al), and case 4 (Heat Sink-6EA, Cooling Channel-Al). To compare the cooling performance for novel design of the smart cooling system, selected local positions for various temperature distributions were marked on the coil surface. Normalized local Nusselt number of the cooling area at the normalized width position indicated that cooling performance of case 1 was on an average 8.05, 0.57, and 5.85% lower than that of cases 2, 3, and 4, respectively.

Forced Convective Heat Transfer Analysis for Two-dimensional Slot Jet of Water-CuO Nanofluid

This paper investigates the effect of the diameter and the volume fraction variation of the centre nanoparticles on the heat transfer characteristics of a two-dimensional slot jet. The jet impinges on stationary flat, convex, and concave aluminium plates. A forced convective heat transfer coefficient of water-CuO nanofluid impinges on a smooth plate under a constant heat flux. The finite volume method (FVM) is implemented for nanoparticles with diameters varying from 7 to 60 nanometers, volume fractions changing from 0 to 5%, and the Reynolds numbers ranging from 1800 to 2800. A grid independence study is carried out to find a grid size that predicts the results accurately and further grid refinement changes the results insignificantly. The single-phase model shows a capability to predicts the fluid and heat transfer parameters faster and make it more suitable for numerical simulations compared to the two-phase model. The results indicate a higher heat transfer coefficient of nanofluid in comparison with distilled water. As the Reynolds number and nanoparticle volume concentrations increase, the heat transfer rate increases on the surface whilst smaller nanoparticle diameters increase during the cooling process. The increase in the diameter of nanoparticles enhances the Nusselt number on the plate by up to 10%. The same geometrical details, thermophysical, and boundary conditions have been employed in all calculations for distilled water jet simulations to validate the fluid flow behaviour and heat transfer parameters with available experimental data in the literature.

Improvement of Condensation Model With the Presence of Non-Condensable Gas for Thermal-Hydraulic Analysis in Containment

Steam condensation plays a key role in prediction of the pressure behavior and hydrogen distribution in the containment during a hypothetical loss-of-coolant accident or a severe accident in a light water nuclear reactor. The objective of this study is to evaluate and improve the condensation model in GASFLOW code. Reynolds analogy coupled with wall function and Chilton-Colburn empirical analogy is used to model heat and mass transfer in GASFLOW, which has requirements for dimensional distance of the first cell near the wall and some deficiencies in description of heat and mass transfer process in the stagnant zone. Based on the evaluation of original condensation, the results shows good agreement with COPAIN experiment cases where the mass fraction of air ranges from 76.7 to 86.4%. However, with the changes in geometry of the facility and the presence of helium, the original model has a large deviation in the prediction of pressure, temperature and gas distribution compared with MISTRA ISP47 (OECD International Standard Problem No. 47) experiment data. This work proposes a modified condensation model which uses McAdams correlation and Schlichting correlation with a weight factor to calculate natural, forced, or mixed convection heat transfer coefficient, and adopts Chilton-Colburn empirical analogy to model mass transfer. The modified model has no requirement for the dimensionless distance near the wall in heat and mass transfer calculation and improves the prediction performance of heat transfer in stagnant zone. The prediction result of the modified model shows good agreements with MISTRA ISP47 problem, and the error of it compared with COPAIN experiment data is within 25% which is the same as that predicted by the original model.

A critical review of filmwise natural and forced convection condensation on enhanced surfaces

The use of enhanced surfaces is an efficient method to improve filmwise condensation. Due to their immense potential, many enhanced structures were developed and investigated with the aim of improving natural and forced convection condensation heat transfer coefficients. This has resulted in large collections of predictive models and experimental data being reported. In this review, the developments in this field of research in the past few decades and the recent advances are collated and examined. This paper focuses on the review of natural convection condensation on the external surfaces of enhanced flat plates and tubes and forced convection condensation in enhanced tubes. The various models predicting the heat transfer coefficients on plain and enhanced surfaces are evaluated. For natural convection condensation, the liquid film-based models, semi-empirical models and numerical modes are reviewed whereas, for forced convection condensation, the gravity-dominated and vapor shear-dominated models are discussed. The effects of these enhanced structures on the liquid film and two-phase flow characteristics are analyzed and the various types of enhanced tubes and flat plates investigated are categorized. The manufacturing techniques employed to fabricate these surfaces are identified. A detailed evaluation of the heat transfer and pressure drop performances of the various enhanced surfaces is performed. In addition, their thermal performances are summarized and compared, and their associated heat transfer mechanisms are elucidated. For external condensation on a single tube row, three-dimensional fin structures were found to provide better thermal performance than two-dimensional structures, with some three-dimensional fin structures exhibiting more than 6 times the heat transfer coefficients of a plain surface. However, in a tube bundle, the heat transfer coefficient of three-dimensional fin tubes decreases more significantly with increasing tube row as compared to two-dimensional fin tubes. For convective condensation in circular tubes, the herringbone and pin fin tubes demonstrated better thermal and pressure drop performances than other internally enhanced tubes. Their efficiency indices were between 1.25 and 1.28. Based on the literature surveyed, the various experimental results are compared, existing research gaps are identified and frameworks for future research work are provided.

Performance assessment of longitudinal flow through rod bundle of ETRR-2 Cobalt/Iridium irradiation device

In the present paper, three dimension CFD analysis was presented for longitudinal flow through rod bundle of ETRR2 Cobalt/Iridium irradiation device. Study of optimal performance for both heat transfer and flow characteristics was performed by the determination of the conditions at which minimum entropy generation exist. The study was performed for Reynolds number (Re) in the range 2 × 104 ≤ Re ≤ 5.5 × 105, and Prandtl number (Pr) in the range 3.2 ≤ Pr ≤ 5.2 under both the conditions of uniform and non uniform (cosine shape) heat flux. Contours of fluid temperature and fluid velocity at irradiation box outlet as well as temperature contours of Ir rods at different values of inlet velocity were illustrated. Two correlations one relating the Nusselt number (Nu) with Re and Pr and the other relating skin friction factor with Re was presented in the studied range of Re and Pr. Determination of the applicability of the existing correlations for turbulent forced convection heat transfer and friction coefficient are also presented. Comparison between the flow and heat transfer characteristics through Ir rods of different locations in the irradiation box is presented.

Experimental investigations on heat transfer to H2O/CO2 mixtures in supercritical region

Supercritical H2O/CO2 mixtures are the working fluids in a novel power generation system with coal gasified in supercritical water, but their heat transfer behaviors have received less attention compared with those of the supercritical water. Here, we experimentally investigated the heat transfer to these important mixtures. First, a stable and accurate apparatus was designed and established to measure the forced convection heat transfer coefficients (HTCs) of supercritical H2O/CO2 mixtures. The measurement uncertainty of this apparatus is lower than those reported in literature attributed to the well controls of the pressure and the component mass fraction throughout the apparatus. Then HTCs were measured for the H2O/CO2 mixtures that flow in a vertically upward smooth tube with the fluid pressure of 24 MPa, temperatures from 400 to 460 °C, CO2 mass fractions up to 20.1% and mass velocities up to 1270 kg m−2 s−1. Results show that the supercritical mixtures have similar convection heat transfer behavior to supercritical pure fluids, and that the forced convection heat transfer coefficients of these mixtures can be well predicted using the correlation developed for supercritical pure fluids. These results lay the foundation for designing heat transfer devices with supercritical mixtures as heat transfer fluids.

Water flow boiling heat transfer in vertical minichannel

In recent years, microchannel water flow boiling has demonstrated excellent heat removal capabilities, particular for nuclear reactor safety utilities, ranging from mini-scale cooling devices to chemical industry plant purposes. Of particular interest is cooling technology using water as the primary refrigerant. This paper presents the results of an experimental investigation into the water boiling heat transfer in a minichannel conduit vertically oriented with the upward flow. The test section was a circular minichannel tube made of stainless steel with a 2.12 mm inner diameter. To observe the flow regime, the test section was equipped with sight tubes made of transparent perfluoroalkoxy alkane resin and Pyrex glass at the inlet and outlet, respectively. Water flow boiling experiments were conducted at a saturation temperature of approximately 100 °C, with the mass flux of water as the working fluid from 20 to 120 kg/m2·s, and the average vapor quality in the test section inlet of approximately 0.05 to 0.9. The predicted two-phase flow regimes at the test section were classified as three patterns, namely slug, annular, and churn flows, using a flow pattern map with the vapor quality as the abscissa versus the mass velocity as the ordinate. The flow patterns were confirmed and visualized by observations at the test section outlet sight tube. The characteristics of the heat transfer and fluid flow were analyzed. Moreover, the forced convective heat transfer coefficient and pressure drop in the test section were determined based on the experimental data. The highest mass flux yielded the highest convective heat transfer coefficient. Despite this significant heat transfer coefficient improvement, the pressure loss was higher than that for a higher mass flux, indicating that the pressure loss was more pronounced owing to the presence of the minichannel at a higher mass flux.

Theoretical investigations on heat transfer to H2O/CO2 mixtures in supercritical region

The supercritical H2O/CO2 mixture is the working fluid to drive a turbine in a novel power generation system with coal gasified in supercritical water. This system is promising because of zero pollution emission in contrast to the conventional coal-fired power plant. Heat transfer coefficients of the supercritical H2O/CO2 mixtures are important to design heat transfer devices in this system, which is similar to the role of heat transfer to supercritical water in conventional systems. However, heat transfer to supercritical mixtures has received less attention. Here, we show that the supercritical mixtures with H2O being one of the components, have similar convection heat transfer behavior to supercritical pure fluids for temperatures and pressures above the critical point of H2O. This phenomenon was demonstrated from two aspects. Firstly, the forced convection heat transfer coefficients of supercritical mixtures were numerically calculated using the simulation model for supercritical pure fluids and using the thermophysical properties (density, heat capacity, thermal conductivity and viscosity) of supercritical mixtures as input. The calculated results agree well with experimental data in the supercritical region. Secondly, the calculated results also agree well with the correlations for supercritical pure fluids. The mechanisms were investigated by molecular dynamics simulations on the diffusion of supercritical mixtures. These results lay the foundation for predicting convection heat transfer coefficients of supercritical mixtures and for designing heat transfer devices with supercritical mixtures as heat transfer fluids.

Globe thermometer free convection error potentials

For thermal comfort research, globe thermometers have become the de facto tool for mean radiant temperature, tr, measurement. They provide a quick means to survey the radiant environment in a space with nearly a century of trials to reassure researchers. However, as more complexity is introduced to built environments, we must reassess the accuracy of globe measurements. In particular, corrections for globe readings taking wind into account rely on a forced convection heat transfer coefficient. In this study, we investigate potential errors introduced by buoyancy driven flow, or free convection, induced by radiant forcing of a black globe’s surface to a temperature different from the air. We discovered this error in an experimental radiant cooling system with high separation of air to radiant temperature. Empirical simulations and the data collected in a radiant cooling setup together demonstrate the influence of free convection on the instrument’s readings. Initial simulation and data show that tr measurements neglecting free convection when calculating tr from air temperatures of 2 K above tr could introduce a mechanism for globe readings to incorrectly track air temperatures. The experimental data constructed to test this hypothesis showed the standard correction readings are 1.94 ± 0.90 °C higher than the ground truth readings for all measurements taken in the experiment. The proposed mixed convection correction is 0.51 ± 1.07 °C higher than the ground truth, and is most accurate at low air speeds, within 0.25 ± 0.60 °C. This implies a potential systematic error in millions of measurements over the past 30 years of thermal comfort research. Future work will be carried out to experimentally validate this framework in a controlled climate chamber environment, examining the tradeoffs between accuracy and precision with globe thermometer measurements.

Determination of Heat Transfer Coefficient for the Air Forced Cooling Over a Flat Side of Coil

Abstract The paper deals with the analytical and experimental determination of the forced convection heat transfer coefficients over the flat coil module. In the analytical part, the forced convection coefficients at different wind speeds are calculated based on various known equations of the forced convection heat transfer coefficient with unheated starting length. The experimental part presents the description of the test: loading the coil with DC current and measurements of the coil temperatures with thermal sensors while it was inside a wind tunnel. Based on the measurement, the convection coefficients were determined. In the final part, the experimental and analytical results are compared. It is found that the accuracy of the analytical results is more precise in highly turbulent flows.

Toward improved heat dissipation of the turbulent regime over backward-facing step for the AL2O3-water nanofluids: An experimental approach

Experimental study of nanofluid flow and heat transfer to fully developed turbulent forced convection flow in a uniformly heated tubular horizontal backward-facing step has reported in the present study. To study the forced convective heat transfer coefficient in the turbulent regime, an experimental study is performed at a different weight concentration of Al2O3 nanoparticles. The experiment had conducted for water and Al2O3 -water nanofluid for the concentration range of 0 to 0.1 wt.% and Reynolds number of 4000 to 16000. The average heat transfer coefficient ratio increases significantly as Reynolds number increasing, increased from 9.6% at Reynolds number of 4000 to 26.3% at Reynolds number of 16000 at the constant weight concentration of 0.1%. The Al2O3 water nanofluid exhibited excellent thermal performance in the tube with a backwardfacing step in comparison to distilled water. However, the pressure losses increased with the increase of the Reynolds number and/or the weight concentrations, but the enhancement rates were insignificant.

Predictive modeling for the cryogenic cooling condition of the hard turning process

This paper presents a numerical model for the hard turning process under the cryogenic cooling condition. This numerical model was developed on the basis of the modified Oxley’s cutting theory with implementing the cryogenic cooling condition. The cooling effect of cryogenic coolant on the tool flank face was modeled as a forced convective heat transfer coefficient as a function of the Nusselt number. The heat generated in the primary and secondary deformation zones was also modeled using moving heat source technique. This model was validated with experimental works under cryogenic and dry conditions for oblique cutting. The minimum and maximum errors in predictions were 1.8 and 15.2% for cutting force (P1), 1.6 and 33.7% for thrust force (P2), and 2.3 and 7.9% for feed force (P3), respectively, under the cryogenic cooling condition. In the case of predicting the temperature at the thermocouple location, the minimum and the maximum errors of these comparisons were 2.0 and 30.5%. It was observed that the cryogenic coolant during the hard turning process reduces the thermal softening effect and in turn increases the cutting forces. In addition, the use of cryogenic coolant can increase the segmented angle (ϕseg) and segmented frequency. Flank wears were observed in both cryogenic cooling and dry conditions. LN2 decreases the length of the flank wear by 12.4~27.5%. In this study, there is the performance improvement of hard turning process by adopting cryogenic cooling method.

Nanofluids and converging flow passages: A synergetic conjugate-heat-transfer enhancement of micro heat sinks

Simultaneous effects of using nanoparticle (Al2O3) in water along with the converging flow passages on the forced convection heat transfer coefficient in a microchannel heat sink (MCHS) are investigated using a finite volume numerical simulation. The accurate KKL (Koo-Kleinstreuer-Li) model, considering particles' material density, volume fraction, diameter and Brownian motion, is implemented to model the thermophysical properties of Al2O3-water nanofluid. The numerical simulation is performed based on a non-uniform structured grid. Results have shown that nanoparticles can enhance the convection heat transfer coefficient of the base fluid and the enhancement obtained by the nanoparticles are 33% higher for the case of the converging flow passages than that of straight passages. Simulation results have proved that implementation of an enhanced working fluid (i.e. Al2O3-water nanofluid) and using geometrical enhancement (i.e. converging flow passages) reveal synergetic thermal response and shows an effective enhancement on the convection heat transfer coefficient as high as 2.35 times greater than the heat transfer coefficient of a pure water flows through a straight channel with no convergence. The present results suggest implementing nanofluids along with converging flow passages to achieve the effective enhancement in the convection heat transfer coefficient and to boost the improvement obtained by each individual enhancement technique, especially in the thermally developed regions wherein the convection heat transfer coefficient cannot be increased by increasing the inlet velocity/pressure in the laminar flow regime.