امیرهمایون مقدادی اصفهانی

In this paper, the dynamic modeling of a D-type boiler was performed during a step increase in loading. The D-type boilers are used in industry to produce process saturated steam. In these types of boilers, heat absorption occurs in two sections of the furnace and the convection section. The pipes of the convection section are indirectly exposed to heat, causes the flow direction to change from the upper drum to lower drum and vice versa. One of the special features of this boiler is that the drum circulation loop is natural and as a result, the hydraulic balance equations of the system must be considered. In the present modeling, the equations of water-side were considered in one-dimensional and the equations of flue gas-side were considered in three-dimensional space using the zonal method. The transient continuity and energy equations of the water-side form a system of nonlinear ordinary differential equations which were solved using the Runge–Kutta method. To solve the water and gas-side equations in two-way coupling method, a Fortran computer program has been developed. Investigation of the boiler operating parameters during start-up can help to improve the existing control system. The comparison of the present results with the test data of the desired boiler showed that maximum modelling errors are equal to 4.8, 1, and 6.8 percent for steam pressure, drum water temperature, and outlet steam of boiler during start-up, respectively. Also, the boiler response to a step increase in consumer steam demand is less than 20 seconds.
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In the present research the effect of adding multi wall Carbon nano tubes (with 10-15 nano meter diameter) on the heat transfer coefficient of a heat exchanger is investigated. This work is performed by a co current flexible double pipe heat exchanger made of PVDF. To this end, the viscosity and thermal conductivity of nano fluids is measured experimentally for various temperatures and concentrations. The nano fluid temperature is set at 35°C, 40°C, 45°C and 50°C. The flow rate of nano fluid inside the inner tube was adjusted at 100 , 200 , 300 while, the cold water flow rate of the outer tube is 100 The results shows that heat transfer coefficient of the heat exchanger improves with increasing the fluid temperature, concentration and flow rate. The most effective factor on the heat transfer coefficient is flowrate. Obtained results indicate that using this nano fluid enhances heat transfer coefficient 75%.

The standard Lattice Boltzmann Method, LBM, is usually able to predict the results of micro scale flows which are corresponding to the slip flow regime, while it has not sufficient accuracy for nano scale flows which are corresponding to the transition flow regime. In this paper, by improving the lattice Boltzmann method, pressure driven flow through non-porous nano channels and nano channels filled with porous media has been modeled for wide range of Knudsen numbers, Kn, covering the slip and transition regimes. The results show that the presented lattice Boltzmann model is able to predict the flow features in micro and nano scales for wide range of Knudsen numbers by modifying the relaxation time. In the presented research, the effects of the Knudsen number and porosity on the flow rate, Darcy number and pressure drop are reported. Also, for the first time, the Knudsen’s minimum effect for micro/nano channels filled with porous media is observed and evaluated. For the pressure driven flows in non-porous channels this effect occurs at Kn=1, while the results show that for the nano channels filled with porous media this effect occurs at Kn=0.1 because of the tortuosity effects.