Jorge L. Alvarado

The study aims at understanding the effects of transversely patterned nonslip and slip (superhydrophobic) surfaces on thermal performance and pressure drop in a circular minichannel under turbulent flow conditions. Numerical simulations of fluid flow through patterned minichannels for a Reynolds number of 5600 under constant heat flux conditions were performed using CFD software Star-CCM+. The κ–ω SST turbulence model with a coupled solver was used for simulating flow through the minichannels. Several numerical cases were simulated to understand the effects of nonslip to slip ratio and width of nonslip bands on pressure drop and thermal performance of circular minichannels. Two nonslip to slip ratios (1 and 1/3) and three different nonslip bandwidths (0.2δ, 0.4δ, 0.8δ) were considered in the simulation process. Parameters representative of the flow and heat transfer behavior such as boundary layer thickness, friction factor, slip velocity, shape factor, Nusselt number, and performance evaluation criterion (PEC) were calculated to better understand the effects of nonslip to slip ratio and width of nonslip bands on thermal performance. The simulation results reveal that a decrease in nonslip to slip ratio and increase in nonslip width lead to a reduction in pressure drop and an enhancement in heat transfer. Furthermore, the configuration with a nonslip to slip ratio of 1/3 and a nonslip width of 0.8δ was found to be the optimal minichannel design, achieving a 40% reduction in pressure drop with a PEC value of 3.4. In summary, the numerical simulations show that properly designed microchannels consisting of slip and nonslip bands arranged transversely to the flow direction can lead to enhanced thermal performance under turbulent flow conditions.

Recent studies have revealed that large eddy simulation (LES) as CFD technique is capable of capturing the unstable features of two-phase swirling flows during the combustion of straight vegetable oil (SVO) emulsions. In this study, blends of canola oil and methanol have been selected as emulsified SVO fuels for simulation purposes. To fully understand the effects of swirl number and equivalence ratio on combustion characteristics of emulsified canola oil, a numerical approach based on the discrete phase model (DPM), stochastic model and Taylor Analogy Breakup (TAB) model have been adopted to simulate SVO blend droplets in a swirl-promoted environment. Moreover, simulation results of fuel droplets distribution are compared directly with experimental results. Furthermore, a two-step chemical reaction mechanism and the Eddy-Dissipation model have been used to understand the effects of swirl number and equivalence ratio on the combustion characteristics of SVO. A robust and stable numerical algorithm has been selected and validated, which is capable of simulating turbulent combustion of emulsified canola oil using LES by considering two swirl numbers and equivalence ratios. Results reveal that turbulent flow and combustion characteristics are affected by the swirl number significantly. The simulation emission data show the same trends as the experimental results even though the simulations depict higher combustion temperatures in absence of heat losses. Nevertheless, the comprehensive numerical approach has been validated using experimental LDA droplet size distribution data. In summary, the numerical approach can be applied as a first order approximation to determine the effects of biofuel blend and swirl number on emission levels.

Microencapsulated phase change material (MPCM) in the form of particles suspended in conventional cooling fluids such as water has shown enhanced heat transfer characteristics when compared to conventional single phase heat transfer fluids. This is attributed to the latent heat of fusion of the MPCM, which results in higher heat capacity of the heat transfer fluid. The thermal performance of MPCM slurry as a heat transfer fluid can be further improved when used in a channel consisting of staggered pins. Use of MPCM slurry in heat transfer channels can result in efficient cooling performance of photovoltaic (PV) systems, where significant thermal loads are prevalent due to high levels of irradiance or working in hot climate conditions. The present work investigates numerically the impact of using MPCM slurry at different mass fractions on the cooling performance of a heat transfer channel attached to a PV panel. The numerical simulations were performed using the ANSYS-FLUENT solver in combination with user-defined functions used to account for the thermophysical properties of MPCM slurry. It was observed that the Nusselt number of MPCM slurry was much higher than for water as single-phase fluid, especially at higher mass fractions at the same flow rate. Moreover, the PV panel temperature was found to be significantly reduced when phase change material in the MPCM slurry experienced melting. Moreover, using staggered circular pins in the channel resulted in a significant reduction of the PV panel temperature even under concentrated irradiance conditions. However, an increase in pumping power was noticeable at high mass fraction of MPCM slurry. Finally, a remarkable increase in the PV panel efficiency was achieved under normal and concentrated irradiance levels by using MPCM slurry in the cooling channel with and without staggered circular pins.

In this study, the flow and heat transfer characteristics of microencapsulated phase change material (MPCM) slurry were experimentally investigated using a newly designed helical coil heat transfer device. The conventional helical coil has been structurally modified with passive enhancement features aiming to further promote fluid mixing. Specifically, 360° plastic tubing with or without wire coil inserts was added after each 180° of the main helical loop to enhance fluid mixing and improve the overall thermal performance of the device. Pressure drop and heat transfer experiments with MPCM slurry were conducted under turbulent flow and constant heat flux conditions. A new friction factor and Nusselt number correlations for MPCM slurry in helical coils with reversed loops and wire coil inserts are proposed. Experimental results show that the structural modifications did enhance the heat transfer performance of MPCM slurry. The experimental results revealed that the phase change process of MPCM considerably enhanced the heat transfer rate of MPCM slurry. Furthermore, the use of reversed loops and wire coil inserts led to better fluid mixing within the coil, resulting in improved convective heat transfer of the MPCM slurry.

An experimental investigation was carried out to study the effects of single and multiple vortex generators (VGs) on the mean velocity, turbulence levels, and surface temperature distributions in a square channel. The flow and heat transfer in the wake of VGs were characterized using particle image velocimetry (PIV) and infrared (IR) thermography. Measurements were performed in the wake regions of VGs, where the counter-rotating vortex pairs (CVPs) were dominant. Inclination angle and taper angle of VGs, spacing-to-width ratio (STW), and streamwise spacing between rows of VGs (S) were varied to understand the effects on flow and heat transfer characteristics. Results reveal a distinct impact of the VGs and layouts on the vortical flow and local convective heat transfer phenomena. The measurements clearly show that configuration parameters such as inclination angle, spacing-to-width ratio, streamwise spacing, and arrangement of multiple VGs are factors in the optimum heat transfer performance applicable to a wide range of thermal management systems.

Solar distillation system has recently been explored and used for the extraction of essential oil from different agricultural herbs. The efficiency of conventional distillation systems is low, and most are driven by hot gases. On the other hand, the solar-driven distillation system has higher efficiency and is based on renewable energy. The modified solar distillation system comprises a Scheffler reflector “concentrator,” newly design steam receiver, distillation still, steam condenser, florentine flask, and biomass-based boiler. A perforated circular pan added at the still bottom and pattern of steam flow and temperature distribution was examined using the computational fluid dynamics (CFD) technique. The CFD simulation results showed that homogeneity in the steam distribution in the distillation still is required to get maximum essential oil extraction efficiency. The energy dissemination at different parts of the newly design solar distillation system was estimated using mathematical modeling. The results of different experiments showed that the yield of essential oils from fresh Rosemary and Cumin was 0.17% w/w and 1.11% w/w, respectively. The efficiency of the modified solar distillation system with a perforated pan and steam receiver was found to be 8% higher than the previous system.