Numerical evaluation of finite length tubes effects in Stirling engines heaters

The performance of Stirling engines is directly related to the amount of energy that, in the form of heat, participates in the thermodynamic cycle. A peculiar characteristic of this type of engine is the closed circuit of the working fluid, implying a periodic admission and extraction, ideally involving the whole working fluid, of the heat exchanged during the cycle without any exchange of material with the external environment. Several correlations have been proposed in the literature to predict the heat exchange in the hot side heat exchangers, but most of them are based on nondimensional numbers, usually expressed in terms of an oscillatory Reynolds number and a non-dimensional length, that even if they take into account the amplitude and frequency of the oscillating flow inside the tube with respect to its diameter, do not include any dependency upon the length of the tube. Nevertheless, the length of the tube can have a great impact on the performance of a Stirling engine. The heater forms a significant part of the dead volume of the engine, making the optimization of the volume to surface ratio necessary. The friction losses increase by increasing the length of the tubes, determining a negative impact while the exchange surface increases. Even the Nusselt number in the inner side of the tubes changes along the length, achieving the largest values alternatively in the first portions of the tube lengths because of entrance effects (Graetz problem). The picture is made even more complex because of the special velocity profiles that develop in oscillating flows in tubes. One of the effects less investigated in previous studies, which we could call a “breathing effect”, regards the amount of working fluid that, in a real engine, can effectively travel from the hot side to the cold side, thus reaching the conditions of nominal heat exchange with the thermal source and sink of the cycle. An idealized configuration has been devised to investigate, using CFD simulations, these effects. Results reporting how the friction coefficient and the Nusselt number depend on the finite length of the tube will be illustrated.

Rationale for vacuum-pulse pump devices applied on cattle farms

One of the main elements to foster technological processes in animal husbandry are vacuumpulse pump devices designed to support dosing, mixing, transporting, lines for forage preparation and feeding, milking cows, milk processing, as well as a large number of technological processes in agriculture. The paper discusses a vacuum-pulse pumping device widely used in all industries and agriculture, a feature of which is to improve technical characteristics of the pump avoiding direct mechanical energy consumption and boasting a fairly simple design. In vacuum-pulse pumping devices, transient events are caused deliberately to increase ejection coefficient, productivity, etc. Oscillating flows of materials being transported are very diverse, due to an increased number of similarity criteria that determine flow patterns. Whereas superficial velocity and the Reynolds number are commonly used for a steady flow, for an oscillating flow, the relative frequency and the relative amplitude of oscillations are added. The objects of experimental research were ejectors with oscillating flows. The wider objective of the experiments was to determine the most effective performance indicators of the ejectors, including the degree of pressure increase, the ejection coefficient and the geometric parameter. Resulting from the experiments, a direct relationship was established between changes in the performance of a pulse-vacuum pumping device and valve material and magnitude of its oscillations. A pulse ejector is recommended to have metal-seated ball valves with a pulsation frequency of 90–100 min−1. Once applied, the proposed pulse ejector will eventually increase the transportation productivity by 14.5 %.

Data reduction of friction factor, permeability and inertial coefficient for a compressible gas flow through a milli-regenerator

A regenerator of a Stirling machine alternately absorbs and releases heat from and to the working fluid which allows to recycle rejected heat during theoretical isochoric processes. This work focuses on a milli-regenerator fabricated with a multiple jet molding process. The regenerator is a porous medium filled with a dense pillar matrix. The pillars have a geometrical lens shape. Two metallic layers (chromium and copper) are deposited on the polymer pillars to increase heat transfer inside the regenerator. We performed experiments on different milli-regenerators corresponding to three porosities (ε = 0.80, 0.85 and 0.90) under nitrogen steady and oscillating compressible flows (oscillating Reynolds number in the range 0 < Reω < 60 and Reynolds number based on the hydraulic diameter ReDh,max<6000) for different temperature gradients (ΔT < 100°C). Temperature, velocity and pressure experimental measurements are performed with microthermocouples (type K with 7,6 µm diameter), hotwires and miniature pressure sensors, respectively. We identified a threeterm composite correlation equation for the friction factor based on a Darcy-Forchheimer flow model that best-fit the experimental data. In steady and oscillating flows permeabilities and inertial coefficients are of the same magnitude order. Inertial coefficients decrease when the porosities increase.

Numerical Investigation on the Transition of Fluid Flow Characteristics Through a Rotating Curved Duct

Time-dependent flow investigation through rotating curved ducts is utilized immensely in rotating machinery and metal industry. In the ongoing exploration, time-dependent solutions with flow transition through a rotating curved square duct of curvature ratio 0.009 have been performed. Numerical calculations are carried out for constant pressure gradient force, the Dean number Dn = 1000 and the Grashof number Gr=100 over a wide range of the Taylor number values –1500 ≤ Tr ≤ 1500 for both positive and negative rotation of the duct. The software Code::Blocks has been employed as the second programming tool to obtain numerical solutions. First, time evolution calculations of the unsteady solutions have been performed for positive rotation. To clearly understand the characteristics of regular and irregular oscillations, phase spaces of the time evolution results have been enumerated. Then the calculations have been further attempted for negative rotation and it is found that the unsteady flow shows different flow instabilities if Tr is increased or decreased in the positive or in the negative direction. Two types of flow velocities such as axial flow and secondary flow and temperature profiles have been exposed, and it is found that there appear two- to four-vortex asymmetric solutions for the oscillating flows for both positive and negative rotation whereas only two-vortex for the steady-state solution for positive rotation but four-vortex for negative rotation. From the axial flow pattern, it is observed that two high-velocity regions have been created for the oscillating flows. As a consequence of the change of flow velocity with respect to time, the fluid flow is mixed up in a great deal which enhances heat transfer in the fluid.

Oscillating Flows in Circular Pipes

Pulsation flows in pipes heated externally produces oscillating temperature field. This type of unsteady flow happens in heat exchangers. Simulating this type of flows is complex in engineering. In this present study the field variables like velocity and temperature are calculated by numerical control volume scheme. Velocity pulsation is applied at inlet of pipe to produce oscillations. Simulation variables like lengths, diameter and thickness of the pipe are considered as parameters for this study. Also additionsl structural constraints has been added to see how it influences effective thermal stresses.