Design of a Jet Impingement Research Setup

Transcription

1 Design of a Jet Impingement Research Setup Design Team Ghanim Al Qassim, Rebecca Meritz, Zach Stebbings, Stefan Tropsa Design Advisor Prof. Mohammed Taslim M.taslim@neu.edu Abstract Jet impingement is an effective way of providing intense, localized cooling in applications where maintaining a proper temperature is absolutely essential to machine function. Jets of a fluid are directed at a heated surface and the flow over this surface provides heat dissipation. Some major applications are the cooling of turbine blades and high performance electronics. In both of these applications, space is limited; especially with respect to the distance, Z, from the jet outlet to the target surface. This design project developed and built a research test setup that can be used to quantitatively measure the heat transfer coefficients on a target wall under different arrays of impinging jets. Many key parameters of the system can be configured: the ratio of the distance Z to the diameter, D; the distance between the center of adjacent jets, P; and the mass flow rate. The final design consisted of a 5x5 array of jets with a P distance that is variable through the use of different jet plates. The Z distance can also be adjusted through the use of different thickness spacers. The jet diameter remains constant at ¼. The entire setup is contained on a mobile cart. In the designed experimental test set-up, air from the plenum impinges on the heated target plate. The temperature gradient on the target plate can be measured from a digital image of a liquid crystal sheet. The reading of this digital image is a technique called liquid crystal thermography; useful in this project for measuring the heat transfer coefficient. The project strictly covers the design and building of the setup. Preliminary tests were conducted to ensure that the setup functioned properly.

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3 The Need for Project The heat transfer properties for different configurations of arrays of impinging jets can be measured with this experimental set up to enable the design of more efficient cooling systems. Impingement cooling by an array of jets is a technique that has applications in a wide range of industries. It is used to efficiently cool a surface, such as in gas turbines, nozzles, combustor linings, shrouds, and electronics. Although, the impingement method has been researched in many scenarios, the majority of the research in this field has been conducted in situations where there is only a single jet and where the ratio of the distance between the jets and the heated target surface, Z, and the diameter of the jet holes, D, is greater than one (Z/D > 1). For most of the technologies described above, an array, not a single jet, is the norm. Also, do too tight space restrictions, a Z/D ratio less than one is needed. The goal of this project is to build a test set-up that enables researchers to measure and understand the heat transfer properties under these realistic conditions. This research will enable future engineers to design superior cooling systems. The Design Project Objectives and Requirements The purpose of the project is to Design Objectives design an experimental set-up The goals of the project were to design and build a completely that can be used to qualitatively self-contained jet impingement test set-up. The system needed to allow analyze jet impingement the user to manipulate the key test parameters: the Z/D ratio, the P/D scenarios with varying P/D and ratio, and the mass flow rate. It is necessary that the set-up design Z/D ratios. allow for free flow of air between jets and target plate because otherwise the measured heat transfer on the heated target plate would be inaccurate. The target plate needed to be heated such that there is a low variability in surface temperature prior to cooling to ensure that accurate heat transfer results are obtainable from the resulting temperature gradients. Design Requirements The number of jets in the array was a critical factor because the cross-flow from adjacent jets affects the heat transfer from each jet. The size had to be large enough to account for this effect, but small enough to make the system a manageable size. The diameter of the jet holes, D, was a constant design parameter. Z and P had to be able to be controlled by the user. Through the use of dimensional analysis, it is determined that the pertinent non-dimensional parameters affecting the

4 impingement heat transfer coefficient are: Re, Z/D, P/D, and Pr. Since the Prandtl number of air is constant, the set-up is capable of varying the other three parameters. The required P/D ratios were 2, 3 and, the required Z/D ratios were 0.3, 0.4, 0.5, 0.6, 0.8, 1, 2 and 3. As the mass flow rate of the system is varied high plenum pressures are reached. Therefore, the systems needed to be able to withstand pressures up to 10 psig. Ideally the target plate surface area would be 90% covered by heating elements to ensure an even heat flux. Design Concepts Considered The overall system design was The set-up consists of three major parts: the plenum, the target provided for the design team, plate, and the cart. For each part there were many design options. however, the design alternatives For the plenum the key considerations were the size, the material, for various components, such as the air hose inlets, the honeycomb material, the instrumentation, and the plenum, the cart, the target the fabrication of the plenum parts. The size of the plenum needed to plate adjustment system, and be chosen to be able to allow for a 5 x 5 array of jet holes. Lexan, heater were considered. Plexiglas, and aluminum were all considered as viable materials for the plenum. The placement of the air hose/inlets was an important design factor for creating an even flow of air. Configurations of one to three inlets on the back and sides of the plenum were considered. The choice of instruments, thermocouples and manometers, and their location is paramount to obtaining accurate experimental data. Furthermore, the decision on how to secure the plenum together was important in ensuring the design was free of leaks. Multiple configurations of bolts, resin, gaskets were considered. To straighten the air multiple honeycomb materials were considered: coffee straws, corrugated cardboard and industrial filters. The target plate design considerations included the connection to the plenum, the material thickness, the heater choice, and the liquid crystal choice. The connection of the target plate to the plenum had to provide free flow of air; some of the suitable options were, set screws, threaded rods, shims, and a self-adjusting actuator. The thickness of the target plate was chosen based on the cost and the deflection. The heaters had strict specifications, to meet the needs of an even temperature profile. Both stock and custom heaters were considered. Different liquid crystals have different temperature ranges and interval

5 ranges. These temperature ranges had to match the temperature profile of the heater. Shelf and custom cart designs were considered. For a custom cart, design considerations included material selection, dimensions, and assembly options. Wood, aluminum extrusion, and Plexiglas designs were considered to balance the needs of cost, strength, aesthetics, and portability. The size and configuration of the cart assembly was based on the plenum size; the needed free space below the target plate; bulitin shelf space for the instrumentation, power supply, and computer; and an adjustable camera stand. Recommended Design Concept The Plexiglas plenum is Design Description connected to the heated target The system was designed to use an existing compressed air flow plate and sits on top of an source. The compressed air is delivered into the plenum through a aluminum extrusion cart, to create hose, through PVC piping, into two inlets on the sides of the plenum. a self-contained, experimental test The plenum has dimensions of 11 x 11 x 12.5 assembled as a set-up. The system allows for free rectangular box made of 1 thick Plexiglas. Inside the plenum the air flow of air, portability, and is straightened by passing through a honeycomb made of corrugated control of Z/D and P/D ratios. cardboard. The air exits the plenum via the jet plate. The lid of the plenum and the jets plates are removable and sealed with a rubber gasket and bolts. The rest of the plenum is secured by bolts and a permanent resin bond. One side of the plenum has 2 J-type thermocouples and a pressure tap attached. Three jets plates were made for the 3 P/D ratios needed with a constant jet diameter of 1/4. The target plate is attached to the plenum at the 4 corners with 4 threaded rods. Spacers went over the rods to adjust the Z distance and were secured by wing nuts. The target plate is made of 1/4 thick Plexiglas. A custom heater is adhered to the front with 3M doublesided tape, and the liquid-crystal sheet is also attached to the back with tape. The heater is made of Inconnel. It consists of an inner main heater which acts as the heated object and surrounding guard heater that keeps heat from escaping out of the sides. The liquid crystal has dimensions of 12 x 12, with a red start temperature of 35 C, and a pressure sensitive adhesive backing. The cart was custom made out of 45mm double and single wide aluminum extrusions secured together with angle brackets and corner

6 blocks. The cart sits on 4 casters. There are two Plexiglas shelves on the cart. The cart has a built in adjustable camera stand. Analytical Investigations The maximum pressure that the plenum needed to withstand was calculated based on based the energy equation and found to be 10 psig. This figure was used in ABAQUS along with the SolidWorks model of the plenum to calculate the maximum stress and deflection of the plenum to ensure they were in safe ranges. This information was used to choose the material. The deflection of the threaded rods was calculated based on their strength and the weight of the target plate to ensure that they could handle the load. The deflection of the target plate itself was also performed in ABAQUS to see if a thinner target plate could handle the pressure of the air flow without much deflection. Key Advantages of Recommended Concept There were many advantages to the design described above. The plenum for instance, was more easily machined, strong, and affordable using Plexiglas. Two thermocouples allow for more reliable temperature measurement. Two air inlets create a suitable mixing of the air. The removable lid was vital to change the honeycomb as well as clean inside of the plenum from any debris. The removable jet plate enabled the control of the P/D ratio. The target plate connection allowed for free flow of air, and experimental repeatability of the Z/D ratio. The target plate is see-through to allow the camera to take pictures of the color change of the liquid crystal sheet. The cart is strong, stable, level, and portable. The design allows for free flow of air between the jet and target plate. Furthermore, it enables the set-up to be one self-contained unit by incorporating the camera stand and shelving for the computer and power supply. Financial Issues The project was constrained due to the expensive cost of the heater The project was financially constrained due to the high cost of the custom heater. Since the heater took the bulk the budget, cost became

7 and aluminum extrusions. The total cost of the project came out to about $1500. a large consideration for the rest of the design. The two other high cost items were the Plexiglas sheets for the plenum and the aluminum extrusions for the cart. Using recycled extrusions greatly reduced the cost of the project. Recommended Improvements A way to improve the system There are a few changes that can be made to improve the design. would be to get the appropriate One of the key design factors was the heater, which when ordered was heater and build a less leak prone supposed to have 90% coverage area. The company that built the system. custom heater for this project claimed to have 98% coverage area, but the delivered product had no more than 60% coverage area. If redesigned a new heater would need to meet this design specifications. Another problem with the design was that the thicknesses of the gaskets were not accounted for, and they are not one continuous piece. These factors cause the plenum to be more prone to leaks. If redesigned the thickness would be accounted for, and they would be cut from one piece of rubber.