Heat and Mass Transfer

Proposing a method for combining monitored multilayered perceptron (MLP) and self-organizing map (SOM) neural networks in prediction of heat transfer parameters in a double pipe heat exchanger with nanofluid Abstract The purpose of this study was to investigate the efficiency of using magnesium oxide nanoparticles, twisted tapes with different and modified twist ratios, and the different rotation velocity of perforated twisted tapes in the evaluation of Nusselt number, overall heat transfer coefficient and pressure drop in a double pipe heat exchanger. Subsequently, the results obtained from the experimental tests of Nusselt number, overall heat transfer coefficient and pressure drop using a modified twisted tape in a double pipe heat exchanger and a back propagation artificial neural network with a multilayered perceptron structure show the existence of a relationship between heat transfer parameters and input data such as cold and hot fluid temperatures, volume fractions of nanofluid, twisted tapes of different twist ratios, the rotation velocity of twisted tapes, Reynolds number of hot and cold fluids, viscosity and thermal conductivity of the nanofluid. Also, the non-monitored artificial neural network selected 33 neurons for use in the multilayered perceptron neural network. The network included the sigmoid function in the hidden layer and the Levenberg–Marquardttraining algorithm with a three-layer topology of 9–33-1 with the correlation coefficients of 0.9987, 0.9933 and 0.9944 and the mean squared error of 0.66535, 4547.68 and 0.008768. This network was best for predicting heat transfer parameters such as Nusselt number, overall heat transfer coefficient and pressure drop.

Numerical investigation of heat and mass transfer behavior of freeze drying of milk in vial Abstract The transient heat and mass transfer characteristics of the freeze-drying process of milk in a vial is numerically investigated. The paper reports the influence of semi- stoppered vial on mass transfer resistance and the relative importance of the bottom curvature of the vial on the drying time. Moreover, the variation of product temperature and mass transfer resistance as a function of time is predicted. The study revealed that the vial heat transfer coefficient strongly depends on chamber pressure; however, the effect of shelf temperature is minimal. The analysis is conducted for two fill heights (8.25 and 16.5 mm.) It is observed that the presence of curvature at the bottom of the vial increases the primary drying time but decreases the product temperature. Based on the simulation, it is observed that 16.5 mm product fill height provided higher mass transfer resistance than 8.25 mm fill height.

A numerical model for wet steam circulating in horizontal wellbores during starting stage of the steam-assisted-gravity-drainage process Abstract Steam-assisted-gravity-drainage (SAGD) has been proved effective in heavy oil recovery. Preheating of the wellbore-surrounding reservoir is to circulate steam in the injector and producer so that heat can be conducted into surrounding oil layer. At this stage, the amount of steam injected into the reservoir is neglected. As a result, creating a large temperature difference between wellbore and annuli is key during the preheating process. A model is established for estimating steam properties in the wellbores so that the highest steam temperature in wellbores can be achieved. The model is comprised of mass, energy and momentum balance equations and the model is solved with numerical method. It is found that: (a) rich heat energy reflected in high steam quality does little effect on heat absorption rate of oil layer. The only effective method for temperature increase in oil layer is to increase the steam temperature in wellbores; (b) in order to increase the heat conduction rate to oil layer, a lower steam quality, a higher steam pressure and a lower mass flow rate is recommended.

Formation mechanism of intermetallic components during dissimilar diffusion bonding of IN718/BNi-2/AISI 316 L by TLP process Abstract In this research, microstructure and intermetallic components of IN 718/BNi-2/316 L joint were studied. Dissimilar bonding was carried out at a temperature of 1050 °C and, bonding time varied at 30 and 45 min in a vacuum furnace under an approximate pressure of 10−5 mbar. After TLP joining, samples were analyzed by energy dispersive x-ray spectroscopy (EDX) and X-Ray diffraction (XRD) methods. After microscopic studies on bonding region and diffusion affected zone (DAZ), it was observed that both types of TLP bonding morphology isothermal and athermal/eutectic are rich in B, Ni, Cr and Fe. On the other hand, the process of intermetallic components formation using ternary phase diagram was studied. Results show that borides components in 316 L – DAZ and IN 718 –DAZ regions were CrB and Nb2Ni21B6, respectively. Additionally, it was observed that intermetallic components texture evolution on the DAZ region was different for In718 and 316 L alloys.

Stationary stall phenomenon and pressure fluctuation in a centrifugal pump at partial load condition Abstract Stall is a common flow phenomenon in the rotating machinery under partial load conditions. The stall phenomenon can seriously affect the operation efficiency and stability of the machinery. In the present research, the stall phenomenon in a centrifugal pump is numerically studied using the SST k-ω turbulence model. In the present work, four different flow rates (1.0 Qd, 0.7 Qd, 0.5 Qd and 0.3 Qd, where Qd is the design flow rate) are investigated, and results reveal that with the decreasing of flow rate, the stall can be divided into the preliminary stall and stationary stall according to the flow structure. When the flow rate decreases to 0.5, the vortexes become strong, but not occupy the whole passage, which is defined as the preliminary stall. When the flow rate further decreases to 0.3 Qd, a fully developed stationary stall appears. Under this condition, the periodic process of stationary stall can be classified into four stages: incepting stage, developing stage, shedding stage and decaying stage. The dominant frequencies of pressure fluctuations under stationary stall conditions are fi, and the maximum amplitudes of pressure fluctuations of PS4 and PS5 at 0.3 Qd are about 5 times that at 1.0 Qd due to the trailing edge vortexes at the blade outlet.

Heat transfer enhancement in a hybrid PV cell-cooling tower Abstract This research has been conducted with the purpose of finding an innovative and environmentally friendly means of cooling for a photovoltaic (PV) solar cell to overcome the common obstacle that hinders PVs to be widely used. A small scale cooling tower was devised and fabricated that works based on evaporation cooling. To optimize the cooling performance, the effect of water flow rate and wind speed were investigated. The experiments were conducted at three different water flow rates with and without considering wind. The average PV cell temperature and the percentage increase in electrical output regarding the reference state of without cooling at each wind speed were measured. After introducing water to the back of the PV cell, the measured value of surface temperature followed a downward trend with a steep slope at lower flow rates. The results showed a significant rise of 44.83% in PV electrical efficiency at the highest flow rate when there is no wind. However, this enhancement reduced to 21.49% and 19.49% for wind speed of 2 m/s and 3.5 m/s, respectively. The net evaporation rate was also calculated for each case to explain the reasons for the observed trends. Finally the feasibility study of the cooling system was conducted and it was found that the proposed cooling system was efficient and practical.

Mass transfer at the confining wall of a coiled flow inverter Abstract An experimental investigation on liquid-wall mass transfer at the internal wall of a coiled flow inverter is carried out. Computation of the mass transfer coefficient is made from measured limiting currents at point electrodes fixed flush with the inner surface of the coiled flow inverter. Variation of mass transfer coefficient with respect to liquid velocity is studied. The coiled flow inverter yielded an enhancement of 125% in mass transfer coefficient against the reported enhancement of 35% in heat transfer. The data were correlated using the least squares regression method and a correlation equation is obtained.

Analysis of natural convection in heat sink using OpenFOAM and experimental tests Abstract A transient three-dimensional natural convection problem in heat sinks with rectangular fins positioned horizontally was studied using the software OpenFOAM (Open Field Operation and Manipulation). OpenFOAM is based on the Finite Volume Method for the discretization of the governing equations and was used to solve the three-dimensional equations of continuity, momentum and energy. These computational simulations were done with the PIMPLE solution algorithm for decoupling the equations. The NusseltCalc tool was used in the post-processing to obtain Nusselt number. The results of Nusselt number, temperature, velocity and vorticity fields were obtained. The temperature results were also obtained by numerical probes and compared with experimental and analytical results; presenting differences lower than 0.7%. The results of the average Nusselt number, \( \overline{Nu} \) , and the average heat transfer coefficient by convection, \( \overline{h} \) , numerically obtained with OpenFOAM were compared with the experimental and with those obtained from empirical correlation. All these results obtained with OpenFOAM presented good accordance with experiments and literature with differences lower than 10%. Uncertainty analyses were also carried out in order to prove the quality of the results and they presented differences lower than 5%. In addition, a Nusselt number correlation is proposed for Rayleight number in the range of 4.6 × 104 < Ra < 5.8 × 105.

Theoretical investigation of thermal wave model of microwave ablation applied in prostate Cancer therapy Abstract This paper presents an hyperbolic Pennes bioheat equation in cylindrical coordinate for modeling the Microwave Ablation (MWA) applied in prostate cancer. Due to recent reports, the number of patients of prostate cancer is growing by 15 million in the world each year. Since, it is shown that application of uniform microwave in prostate area of different patients may produce different temperature, the Pennes bioheat equation is considered to study the effect of perfusion term on produced temperature profiles. The solution method is Eigen value method which results in a closed form solution. The hyperbolic behavior of temperature profiles under high heat release against Fourier model is shown. Results show the importance of tissue perfusion term in estimation of temperature profiles and establish that the thermal tissue damage is expected to initiate from 1 to 3 mm above the catheter surface and to promote up to 7 mm. The solutions can be applied as a verification branch for other numerical works and can be very useful to reduce uncertainty about MWA treatments and improve the reliability of clinical protocols giving insight to the Surgeons.

Experimental study on the convective heat transfer performance and pressure drop of functionalized graphene nanofluids in electronics cooling system Abstract Cooling of electronic equipment has gained significant importance in thermal management systems as a result of the increase of power densities in micro-electronic equipment. This present work investigates the convective cooling performance of microwave assisted acidic functionalized graphene – deionized water nanofluid. The nanofluid possess high dispersion stability (Zeta potential < −40 mV) in the pH range (6–8) and a thermal conductivity enhancement of 55.38% compared to base fluid. The heat transfer performance of the nanofluid was studied by investigating the effect of volume fraction (0 to 0.2 vol.%) and flow rate (5 ml/s to 10 ml/s) on the convective heat transfer coefficient, processor core temperature, and pressure drop. An increase in the convective heat transfer coefficient of about 78.5%, a decrease in the core temperature of about 15% and an average increase of 5% in pressure drop were obtained for the maximum concentration and flow rate of the nanofluid. The results concluded that acidic functionalization of graphene nanoparticle has a significant influence on the increase in the thermo-fluid properties of nanofluid and thus can be used as an efficient heat transfer fluid, compared to conventional coolants.