Maximum Performance Evaluation Criterion Research Articles

Numerical simulation of convective heat transfer of nonhomogeneous nanofluid using Buongiorno model

The aim is to study the assessment of the flow and convective heat transfer of laminar developing flow of Al2O3–water nanofluid inside a vertical tube. A finite volume method procedure on a structured grid was used to solve the governing partial differential equations. The adopted model (Buongiorno model) assumes that the nanofluid is a mixture of a base fluid and nanoparticles, with the relative motion caused by Brownian motion and thermophoretic diffusion. The results showed the distribution of nanoparticles remained almost uniform except in a region near the hot wall where nanoparticles volume fraction were reduced as a result of thermophoresis. The simulation results also indicated there is an optimal volume fraction about 1–2% of the nanoparticles at each Reynolds number for which the maximum performance evaluation criteria can be obtained. The difference between Nusselt number and nondimensional pressure drop calculated based on two phase model and the one calculated based on single phase model was less than 5% at all nanoparticles volume fractions and can be neglected. In natural convection, for 4% of nanoparticles volume fraction, in Gr = 10 more than 15% enhancement of Nusselt number was achieved but in Gr = 300 it was less than 1%.

Performance evaluation of multi-layered porous-medium micro heat exchangers with effects of slip condition and thermal non-equilibrium

Porous-medium micro heat exchanger can be employed for dissipating heat in high heat flux applications. In this work, thermal performance of multi-layered micro heat exchangers with porous medium is theoretically studied. Forced convective heat transfer in porous microchannel is analytically investigated by considering the velocity/thermal slips at the wall. With the local thermal non-equilibrium (LTNE) model, analytical solutions for velocity and solid/fluid temperatures are obtained. Thermal resistance network method is presented for thermal transport in porous micro heat exchangers with the temperature jump thermal resistance. Based on the analytical solution, thermal performance of multi-layered porous-medium micro heat exchangers is evaluated for counter-flow (CF) and parallel-flow (PF) arrangements in terms of the local temperature distribution, log mean temperature difference, and effectiveness. It is found that the Nusselt number of porous microchannel first increases and then decreases with an increase in Knudsen number. The performance evaluation criteria (PEC), integrating flow resistance and heat transfer, is employed for evaluating thermal performance of porous microchannel. A maximum PEC, corresponding to an optimal pore density, exists with the increased pore density. Thermal performance of micro heat exchanger with porous medium is much better than that without porous medium, and the CF heat transfer is superior to the PF heat transfer. This work is useful for analysis in porous microchannels and design of micro heat exchangers with porous materials.

An experimental investigation of heat transfer enhancement in minichannel: Combination of nanofluid and micro fin structure techniques

This work experimentally studied the single-phase heat transfer and pressure drop characteristics by using two heat transfer enhancement techniques (micro fin structure and nanofluids) in multiport minichannel flat tube (MMFT). MMFT consisted of numerous parallel rectangular minichannels and is widely used in industry as the heat transfer unit of a heat exchanger. Firstly, the enhanced heat transfer performances by individually using one enhancement technique were investigated by testing Nusselt number, friction factor and performance evaluation criterion (PEC). In this section, five MMFTs with different micro fin numbers (N=0, 1, 2, 3 and 4) and nanofluids with three volume concentrations (φ=0.005%, 0.01% and 0.1%) were used as test sections and working fluids respectively. Secondly, the experiments using two combined enhancement technique were performed. By using conjunctively two enhancement techniques, Nusselt number increases by up to 158% at about Re=3600 and the maximum PEC value can reach 2.0 at Re=5150. Finally, an optimal heat transfer scheme was proposed based on test data.

An empirical study on vortex-generator insert fitted in tubular heat exchangers with dilute Cu–water nanofluid flow

The heat transfer enhancement (HTE) in tubular heat exchangers fitted with vortex-generator (VG) inserts is experimentally investigated. The studied four parameters and ranges are: winglets-pitch ratio (1.33, 2.67, and 4), winglets-length ratio (0.33, 0.67, and 1), winglets-width ratio (0.2, 0.4, and 0.6), and Reynolds number (5200 to 12200). The testing fluids are the water and Cu–water nanofluid at the volumetric fraction of 0.2%. The results obtained on HTE, pressure drop, and performance evaluation criterion (PEC) are compared with those for water in a smooth tube. It is found that the VG inserts with lower winglets-pitch ratio and higher winglets-length/width ratios present higher values of HTE and pressure drop. Over the range studied, the maximum PEC of 1.83 is detected with the Cu–water nanofluid inside the tube equipped with a VG insert at the winglets-width ratio of 0.6 for the maximum Reynolds number, when the heat transfer rate and pressure drop are 1.24 times and 2.03 times of those in the smooth tube. Generalized regression equations of the Nusselt number, friction factor, and PEC are presented for the tubular heat exchangers with the VG inserts for both water and Cu–water nanofluid. It is concluded that the main advantage of the VG inserts is their simple fabrication and considerable performance, particularly at higher Reynolds number.

Numerical studies on thermo-hydraulic characteristics of laminar flow in a heat exchanger tube fitted with vortex rods

An analysis of the thermo-hydraulic characteristics of a heat exchanger tube fitted with a novel vortex rod insert has been conducted numerically under uniform heat flux conditions. The vortex rods had three relative rod length ratios (LR = b/D = 0.15, 0.25, and 0.35) and three relative rod pitch ratios (PR = p/D = 1, 1.5, and 2), and they were studied for numerous Reynolds numbers (Re) that ranged from 300 to 1800 using water as the working fluid. With these vortex rods, longitudinal swirling (or vortex) flow can be generated for better fluid mixing between the wall and core flow regions, thereby allowing for a high heat transfer rate with a moderate increase in the pressure drop. The results indicate that the presence of the vortex rods elicits a considerable heat transfer enhancement in the tube, Nu/Nu0 = 1.1–3.9, with an increase in pressure drop, and f/f0 = 1.4–5.3. Both the Nu/Nu0 and the f/f0 ratios increase with Re and LR increases and with PR decreases. The performance evaluation criterion (PEC) values of all vortex rod cases are above unity except for the cases with LR = 0.15 at Re = 300. The case at which LR = 0.35 and PR = 1 provides the maximum PEC value of approximately 2.22 at Re = 1800. These results provide evidence that the vortex rod insert is a promising technique for laminar convection heat transfer enhancement.

Numerical study of nanofluid forced convection flow in channels using different shaped transverse ribs

A numerical investigation is performed to study the effects of different rib shapes and turbulent nanofluid flow on the thermal and flow fields through transversely roughened rectangular channels with Reynolds number ranging from 5000 to 20000 and uniform heat flux of 10kW/m2. Considering single-phase approach, the two-dimensional continuity, Navier–Stokes, and energy equations were solved by using the finite volume method (FVM). The optimization was carried out by using various rib shapes (rectangular shape, triangular shape, wedge pointing upstream, and wedge pointing downstream) in two arrangements (in-line and staggered) and three different aspect ratios (w/e=0.5, 2, and 4) to reach the optimal geometry with maximum performance evaluation criterion (PEC). The main aim of this study is to analyze the effects of nanoparticle types (Al2O3, CuO, SiO2, and ZnO), concentration (1–4%), and nanoparticle diameter (30–80nm), on the heat transfer and fluid flow characteristics. Simulation results show that the ribbed channels' performance was greatly influenced by rib shapes and their geometrical parameters. The highest PEC was obtained for the in-line triangular ribs with w/e=4 at Re=5000. It is found that the water–SiO2 shows the highest heat transfer enhancement compared with other tested nanofluids. The Nusselt number through the ribbed channels was enhanced with the increase of the particle volume fraction and Reynolds number, and with the decrease of nanoparticle diameter.

An experimental study on vortex-generator insert with different arrangements of delta-winglets

Heat transfer enhancement in a tube using the VG (vortex-generator) insert with different arrangements of the delta-winglets is investigated. Fourteen VG inserts with the longitudinal and forward arrangement of the delta-winglets are made from the aluminum sheets with a length of 350 mm, a width of 14.5 mm, and a thickness of 0.6 mm. The heat transfer and pressure drop results achieved from the use of the VG inserts inside the tube are compared with those obtained for the plain tube. It is found that at the transitional flow through the plain tube, Notter-Rouse equation predicts the current experimental Nusselt number better than Gnielinski equation. Also, the experimental results reveal that the use of the VG inserts inside the tube yields higher heat transfer coefficient and pressure drop than the plain tube, and these parameters augment with increasing the delta-winglets. The appropriate tradeoff between the enhanced heat transfer and the friction is found by using a special arrangement of the delta-winglets on the VG insert which presents the highest heat transfer coefficient as well as the maximum values of considered PEC (performance evaluation criterion). The maximum PEC of 1.41 is found for this VG insert at Re = 8715.

An experimental study of Cu–water nanofluid flow inside serpentine tubes with variable straight-section lengths

Indirect channels like the serpentine tubes are widely utilized in many engineering applications such as chemical and petrochemical industries, air conditioning and refrigeration systems, and modern energy conversion. Nanofluids are advanced and potential coolants, which can provide appropriate thermal performance in heat exchange devices. In this paper, fluid flow and heat transfer characteristics of Cu–water nanofluid inside five serpentine tubes with variable straight section lengths are experimentally investigated. The concentrations of 0%, 0.1%, and 0.4% wt. of stabilized Cu–water nanofluid are examined with variation of flow rates in the range of 1–5l/min. The Cu–water nanofluids are produced by a one-step method, namely electro-exploded wire (EEW) technique, and the thermo-physical properties of the nanofluids required for the analysis are systematically measured. To obtain accurate results, a highly precise test loop with the ability to produce a constant wall temperature condition is designed and fabricated. It is found that creating short straight section lengths at the beginning of the serpentine tubes enhances both the heat transfer coefficient and pressure drop values. However, this technique improves the overall thermal–hydraulic performance of the serpentine tubes about 10%. Also, the results show that Cu–water nanofluids flow inside all the studied serpentine tubes increase the heat transfer rate. The maximum performance evaluation criterion (PEC) of 1.18 is obtained for the 0.4% wt. nanofluid inside the serpentine tube with Low to High straight section lengths.

Effect of nanoparticle shapes on the heat transfer enhancement in a wavy channel with different phase shifts

A numerical investigation is performed to study the effects of different nanofluids on the thermal and flow fields through transversely wavy wall channels with different phase shifts between the upper and lower wavy walls. Reynolds numbers are considered in the turbulent range of 6000≤Re≤18,000 and a uniform wall temperature of 400K is applied on the walls. The two dimensional continuity, Navier–Stokes and energy equations are solved by using finite volume method (FVM). The optimization was carried out by using various phase shifts (θ=0°, 30°, 60°, 90° and 180°) and three different wavy amplitudes (α=0.5, 1 and 1.5mm) to reach the optimal geometry with the maximum performance evaluation criterion (PEC). The main aim of this study is to analyze the effects of SiO2 nanoparticles, its concentration (1–4%), and nanoparticle shapes (i.e. blades, platelets, cylindrical, bricks, and spherical), on the heat transfer and fluid flow characteristics. Simulation results show that the wavy channel performance was greatly influenced by changing the phase shift and the wavy amplitude. The highest PEC was obtained for the phase shift of θ=30° with α=0.5 at Re=6000. It is found that the SiO2-EG nanofluid with platelets nanoparticle shape gives the highest heat transfer enhancement compared with other tested nanofluids.

Heat Transfer Enhancement Using Nanofluids in a Circular Tube Fitted with Inserts

A numerical investigation is performed to study the effects of different geometrical parameters and nanofluids on the thermal and flow fields through circular tubes fitted with inserts with Reynolds number ranging from 500 to 1750 and heat flux of 1000 W/m(2). The two dimensional continuity, Navier Stokes and energy equations are solved by using the finite volume method. The optimization through twisted tape with eleven types of tube having 20 mm inner diameter was studied by changing the dimensions of twisted tape such as the width ratio (W/D) in the range of 0.2-0.8 and the twist ratio (H/D) in the range of 2.5-4 for water as a working fluid to reach the optimal geometry with maximum performance evaluation criterion (PEC). The effects of using different types of nanoparticles such as Al2O3, CuO, SiO2, and ZnO, different nanoparticles volume fractions in the range of 1% to 4% and different nanoparticles diameters in the range of 25 nm to 80 nm in water base fluid were examined. The effects of different Reynolds numbers and the dispersion of nanoparticle in different types of base fluid such as ethylene glycol, engine oil and water in circular tube were also analyzed. Simulation results show that the Nusselt number increased as the width ratio and Reynolds number increased and twist ratio decreased. The highest Nusselt number was obtained with W/D = 0.8, while the efficient flow (max. PEC) was achieved with W/D = 0.7 for H/D = 2.5. The Nusselt number through the circular tubes with insert was enhanced with the increase of the particle volume fraction and with the decrease of nanoparticles diameter.

Influence of geometrical parameters and forced convective heat transfer in transversely corrugated circular tubes

A numerical investigation is performed to study the effects of different geometrical parameters on the thermal and flow fields through transversely corrugated circular tubes with Reynolds number ranging from 5000 to 60,000 and heat flux of 50W/cm2. The optimization through nineteen transversely corrugated (roughened) tubes having 10mm inner diameter was studied by changing the dimensions of roughness such as relative roughness height (e/d) in the range of 0.025–0.1, relative roughness pitch (p/d) in the range of 0.5–1.5 and relative roughness width (w/d) in the range of 0.05–0.2. For water as a working fluid, heat transfer can be increased with optimal roughness dimensions and maximum performance evaluation criterion (PEC). The results show that the corrugated tube performance was greatly influenced by the geometrical parameters. The Nusselt number increased as the roughness height, width and Reynolds number increased and with the decrease of roughness pitch. The highest Nusselt number was obtained with e/d=0.1, while the efficient flow (max. PEC) was achieved with e/d=0.025 for p/d=0.5 and w/d=0.2. For the rib-roughened tubes, the influence of Reynolds number on the friction factor is quite weak while it is increased with the increase of roughness height compared with the smooth tube.