The application of this hybrid technique requires:correlations for the eddy momentum and heat diffusivity that take into account the strong convective coolant mixing process induced by the wire wrap;prior knowledge about the location of the hot spot in order to avoid costly calculations at all axial planes;extensive tests to the developed codes with comparisons of the predicted results with local experimental data. Experimental data for the velocity field can be found in several previous works see, for example, Roidt et al.
Also, Lafay et al. To our knowledge, no experimental data for the wall shear stress distribution, in wire-wrapped rod bundles have been published. The purpose of the present work is to present experimental results for local static pressure and wall shear stress distributions in turbulent air flow.
The main ideas are to provide some insight about the fine structure of the flow in the subchannels and to furnish local data of the fundamental flow parameters. These data can be used for comparison with the predicted results given by the distributed parameter codes. The measurements were performed in an open loop with air as the flowing fluid. Figure 1 shows a general view of the apparatus. The flow rate was measured by a calibrated Pitot tube mounted in the feeding tube and it was controlled by a throttling valve located adjacent to the fan.
The test section consists of a bundle assembly with 7 wire-wrapped rods. Flow perturbations due to the instrumentation were reduced by taking rods with diameter of 50 mm.
The full length of the test section is mm. The static pressure profiles at wall of the hexagonal duct were obtained by 9 pressure taps uniformly distributed on one side of the housing as shown in Figure 2. The hole diameter is 0. To measure the static pressure and the wall shear stress distributions on the surface of the rods, a static pressure take and a Preston tube were installed on a portion of one of the rods, as presented in Figure 3.
Such section can rotate independently of the remaining part of the rod and of the wire. This procedure allows continuous angular measurements of the parameters. The static pressure take has a diameter of 0. The outside diameter of the Preston tube is 0. From axial pressure drop measurements, the flow was observed to be fully developed at that stage. The Preston tube was calibrate with a TSI calibrator, model OR and the experimental data were reduced using the results given by Patel The angular wire position is given by the angle a, defined in Figure 2 , and its effect on the measured profiles was experimentally observed by rotating simultaneously the seven rods of the same amount.
The static pressure distributions on the wall of the hexagonal duct were measured for a varying in steps of 60o and for Reynolds numbers, Re, between and For the central rod, the static pressure and the wall shear stress profiles were obtained for a varying in steps of 30o, while for the peripheral rod, it was considered a varying in steps of 60o.
The shear stress readings were non-dimensionalized dividing them by the average value of the wall shear stress, to,avg, obtained from the friction factors previously measured in pressure drop measurements. For side A in Figure 4 , the wire is in the gap between the rods of the measuring face and is leaving the edge subchannel.
It can be observed that the wires induce a low pressure zone in their wake region. For side D, the wire is also blocking the gap between the rods but, in this case, it is entering the edge subchannel, directing the flow against the wall and creating a high pressure zone. When the wire is in the edge subchannel, as for side B, the static pressure has a very low value near the left side of the wall, which is the wake region of the wire of the left rod.
In this case, the wire is almost blocking the gap between the wall and the rod. Similar results can be observed in Figure 5. These measurements are in qualitative agreement with the result of Lafay et al , obtained for 19 wire-wrapped rod bundle. Measurements of the static pressure distributions are presented in Figures 6 and 7 , for the central rod, and in Figures 8 and 9 , for the peripheral one. In the cases shown in Figures 6 and 8 , the surface of the rod is in contact with the wire of a neighboring rod.
Maximum and minimum static pressures are observed on both sides of that spacer. Stress is the measure of an external force acting over the cross sectional area of an object.
The SI units are commonly referred to as Pascals, abbreviated Pa. There are two types of stress that a structure can experience: 1. Normal Stress and 2. Shear Stress. When a force acts perpendicular or "normal" to the surface of an object, it exerts a normal stress.
When a force acts parallel to the surface of an object, it exerts a shear stress. Let's consider a light fixture hanging from the ceiling by a rope. The cross section of the rope is circular, and the weight of the light is pulling downward, perpendicular to the rope. This force exerts a normal stress within the rope. Okay, how did we arrive at this equation.
There are a lot of assumptions behind the scenes. Throughout this course, we will assume that all materials are homogenous , isotropic , and elastic. We will also assume that the object is "prismatic" — meaning the cross sections are the same all along its length e.
All these assumptions allow us to state that the object will deform uniformly at every point of its cross section. The normal stress at a point on a cross section is defined as with similar equations in the x and y directions. Each small area of the cross section is subjected to the same force, and the sum of all these forces must equal the internal resultant force P.
This relationship for the normal stress is more accurately an average normal stress , since we've averaged the internal forces over the entire cross section. As it turns out, placing a transparent object through cross polarized light allows you to directly observe stress within a material, based on a concept called photoelasticity :. Quite confusing, right? We will proceed with the major difference in pressure and stress, following the real-life examples, and further explain these two in detail.
Stress applies at the internal level. The pressure applies externally to the surface. For example, pressure on the ball or pressure on the surface of the fluid. Stress acts on the solids only. Pressure occurs on liquids, fluids, and gases.
Stress is of two types viz: normal and shear. Shear stress is a force applied parallel to the block. Pressure occurs normally on the surface. For example, the normal pressure difference in the airfoil creates a lift.
Stress is a vector quantity. It means stress has both magnitude and direction both. However, one more term adds to this physical quantity and that is the point of application.
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