مجله مهندسی مکانیک امیرکبیر
سال پنجاه و پنجم شماره 3 (خرداد 1402)

In this research, a meshless method has been developed to solve internal and axi-symmetric flows. In this method, the least squares of the Taylor series are used for spatial discretization and explicit multi-step Runge-Kutta method is used for temporal discretization. Governing equations are based on two-dimensional and axi-symmetric Euler equations. The second and forth order dissipation are used to solve the flows. In order to model boundary condition, subsonic and supersonic inlet and outlet boundary conditions as well as the wall boundary have been used according to the problem. To validate the results of the code, the inviscid flow inside a two-dimensional nozzle and the supersonic flow inside the channel along with bump have been simulated and the results have been compared with valid data. Also, the ability of the code to shock capturing in the two-dimensional and axi-symmetrical nozzle is presented. Finally, the simulation of the steady flow inside a axi-symmetric convergent-divergent supersonic nozzle with Mach 5 in outlet has been done to measure the accuracy of solving the numerical code at hypersonic speed. The results show that the developed code can simulate steady internal and axi-symmetric flows with very good accuracy. The code convergence process is also presented, which shows appropriate convergence of the developed code. The analysis time for shock capturing in the axisymmetric nozzle is about 64% faster than the Fluent-software.

In this article, application of heat pipe to improve dehumidification in air-handling units (AHU) used in hot and humid climate conditions of north of Iran is studied. Coupled calculations of direct expansion (DX) coil and wrap around heat pipe (WAHP) heat exchanger are presented. Two types of buildings, i.e. with 100% and 25% fresh air are considered. Performance of AHU equipped with WAHP is compared with that of conventional AHU. The calculation results are compared and validated with outputs of an online software of a reputable WAHP manufacturer. For economic analyses, two scenarios are considered; first, WAHP is installed as a part of a brand new AHU in the factory or, second, WAHP is installed as a retrofit on an existing and operating AHU. Results show that 5% and 25% of electricity savings are obtained by using WAHP on AHU of buildings with 100% and 25% fresh air, respectively. Also, apart from the energy saving issue, the brand new AHU equipped with WAHP is more than 20% cheaper than the conventional AHU. For energy tariffs, two perspectives are considered: the consumer perspective (including significant subsidies) and the governmental perspective (including no subsidy). From a governmental perspective, adding a WAHP to the existing and operating AHU is profitable. For example, in a scenario, an IRR of 45% and a 5 years investment return was obtained.

The impinging jet array is capable of enhancing heat transfer rates over the impinging surface. A sever challenge of such cooling method is that it does not transfer the heat across the target plate as uniformly as required in many applications. The main objective of the current numerical research is to study the effects of covering the impingement surface by a porous layer in order to increase the heat uniformity throughout the surface without losing the advantage of high heat transfer rates in a multiple impinging jet flow. The porosity, permeability and thickness of the porous layer are considered. The standard deviation of local Nusselt numbers compared to the average value is used as a measure to evaluate the uniformity of heat transfer along the plate. A performance evaluation factor is proposed to simultaneously account for uniformity as well as total rate of heat transfer. The results show that lowest evaluation factor of 0.137 is obtained once the flow channel is fully covered by the porous medium and the permeability and porosity are at their lowest values of k=1.76*10-12 and ԑ=0.2, respectively. The highest heat transfer performance is obtained for the case when the channel is half filled with the porous layer and the coefficient of permeability and porosity are at their lowest (k=1.76*10-12) and highest (ԑ=0.8) levels, respectively.

Fuel cell is a type of electrochemical energy converter that converts chemical energy stored in fuel into electrical energy. Nonlinear structure, time-varying dynamics and uncertain physical parameters are the challenges of working with polymer electrolyte membrane (PEM) fuel cells. In this paper, grey-box modeling and system identification of flow-through H2/O2 PEM fuel cell with three cells and integrated humidifier is investigated. First, zero-dimensional nonlinear fluidic, thermodynamic and electrochemical modeling of PEM fuel cell is performed. The fuel cell model presented in this research is Multi-Input-Single-Output type. In the following, constant parameters of the studied fuel cell are determined. The system identification process as a multi-input-single-output system is done based on the Prediction-Error minimization, using the method of Trust-Region Reflective Newton. Finally, validation of the obtained model is done with experimental data that not used in modeling. The considered PEM fuel cell was tested under different conditions of temperature, reactant gas inlet pressure and stack current, and 329,420 experimental data were obtained. According to test conditions, data was classified into 189 different modes. The results show that the average voltage error of the identified model compared to the experimental data is equal to 1.03%. Moreover, the correlation between voltage and identified model parameters has been investigated, and the results showed that correlation between voltage and the contact resistance equivalent is higher than coefficients of orifices.

One of the unique features of viscoelastic fluids in unsteady flows is the damping oscillatory behavior without using oscillation and external force, the cause of this is the elastic feature of these fluids. In this essay, the power-law preconditioning method of local stress sensors is used for the first time, to solve the numerical solutions of unsteady flows of viscoelastic fluid while passing through two fixed parallel planes. The Maxwell's equations have been used for this simulation. In this method, by adding a false time derivative to the governing equations, the formation of equations become hyperbola (artificial compressibility). By acquiring the preconditioning matrix of these equations that are corrected from the relationship between stress fields, solving unsteady incompressible flow equations as artificial compressible, is possible when using a dual-time algorithm that contains inner and external cordon. For convergence of inner cordon , Vossougi Far’s four-step numerical method is used. For discretization of equations, the first-order finite difference and second-order finite difference are used. The calculations of unsteady flows of viscoelastic fluid for Reynolds numbers, Weissenberg numbers and various passive viscosity ratio, are presented. These results are similar to numerical results. The convergence rate results show that the power-law preconditioned method of local stress sensor for a viscosity ratio of less than 0.5 for high stability, increase the convergence speed and reduce the cost of calculations.

In this research, the effect of various operating conditions on the performance of an Unmanned Aerial Vehicle (UAV) with a PEMFC propulsion system is surveyed using a statistical approach, namely the Design of Experiment technique (DOE). This process starts by designing a thermodynamic cycle for UAV based on the literature review. Then, a zero-dimensional model is established to simulate the PEMFC’s performance. After validating the developed model for PEMFC, mass and energy balance equations are employed for all components of the thermodynamic cycle to calculate the efficiency of PEMFC and the system simultaneously. Afterward, the Response Surface Method (RSM) is used to design appropriate tests to study the influence of independent variables, i.e., altitude, operating pressure, and cathode stoichiometry, on the efficiency of PEMFC and system. Results indicate that increasing operating pressure improves the efficiency parameter for both the PEMFC and the system 3.5% and 14.5% respectively. Although increasing cathode stoichiometry augments the PEMFC efficiency, it plummets the system efficiency up to 30.6%. In addition, the influence of operating altitude on the PEMFC efficiency is negligible, while it causes a substantial decline in the system efficiency (more than 30%). As proved, at any desired operating altitude, maximum efficiency for the system obtains when the operating pressure and cathode stoichiometry are set at their maximum and minimum bound, respectively.