Proceedings of

.MAE 585, Solar Thermal Engineering, spring 14 Department of Mechanical Engineering, Arizona State University May 2, 2014, Tempe, AZ, USA

COOLING OF ELECTRONIC EQUIPMENT

Navya Gundeti, Anand Sethuramu, Rutuja Bhadre, Santosh Pattanad, Pushkar NaikSchool for Engineering of Matter, Transport, and Energy Arizona State UniversityTempe, AZ, USA

7ABSTRACTThe methods of cooling of electronic equipment were studied and their mechanisms analyzed. Jet impingement technique was selected for performing experiment and analyzing the cooling patterns on the copper plate, which acts as a prototype for heated electronic equipment. The experiment was conducted on four cases, varying the H/D ratio and the distance of stagnation point from the center. The data collected was tabulated and graphs with the same data were plotted. Further a CFD simulation was carried out to cross verify the experimental result.

NOMENCLATURE

L=characteristic length V=voltage in volts Pm=Mean power loss T = waveform period i(t)=instantaneous value of current through the element v(t)=instantaneous value of voltage through the element t1 = lower limit of conduction for the current t2 = upper limit for the current P = power in watts or joules/sec I = current in Amps T=temperature difference between the solid surface and surrounding fluid area H = distance between nozzle and surface D = diameter of jet impinging nozzle

INTRODUCTIONElectronic systems cooling have been extensively examined recently because of the interest from both scientific and technological points of view. In the technological part, with the improvements of the electronics systems, higher processing speeds, more power, smaller systems become more of a necessity than ever before. One of the most important results of these necessities is the need to handle more complex geometries. Therefore one of the biggest issues in the electronics systems is the cooling of these complex geometries. Especially in military, healthcare and aerospace applications,

effective cooling is crucial. With an efficient cooling, electronics systems become more reliable and durable [1].On the scientific part, heat that is generated in electronic systems is proportional to the square of voltage and the frequency of the system. So, engineers try to increase the frequency and decrease the voltage to be able to decrease heat generation. Developing effective cooling techniques of electronic systems is a major challenge. Therefore different cooling techniques have been developed to remove the heat from electronic components.

Major Methods of Cooling Major methods of cooling were considered and a market research was done on them: Conduction Cooling: It is a cooling system mainly used for harsh-environment applications where conventional, air-cooling techniques cannot be used. Its study revealed that increasing number of military applications require conduction cooling where systems were sealed. It can be noted that Heat spreading is a very effective way of mitigating the need for sophisticated high-heat flux cooling options. Research shows that conduction cooling is based on the diffusion of heat through a solid, liquid, or gas as a result of molecular interactions in the absence of any bulk fluid motion [2]. Conduction cooling can be carried out using: Metal pads and Heat pipe. Thermal Conduction Module: Innovative heating technique introduced, by IBM for 3081 Processor Unit [2]. Unlike conventional methods, IBM has come up with TCM concept. Analysis shows that in TCM, one side of the chip is reserved for electrical connections and the other side for heat rejection. It was observed that the chip was cooled by direct contact to the cooling system to minimize the junction-to-case thermal resistance. Piezoelectric Cooling: Market research on this revealed that once an exchanging current passes through a ceramic segment, it grows and contracts at up to 150 times each second so the nickel plates act like a howls [4]. It was seen that at the point when the piezoelectric material chokes, the edges of the two-nickel circles were pushed together so they twist far from one another and suck in hot air from the encompassing range. At that point, when the piezoelectric material extends, the nickel circles meet up and the air was casted out from the core at high speed, which resulted in cooling. Phase Change Material: It is a very safe method of cooling which uses a Heat Storage Unit HSU, consisting of a Phase change material. Studies show that it is a technique used for limiting temperature increases and/or minimizing the performance requirements of a heat sink when electronics are operated under transient conditions, increasing the thermal capacitance [3]. It was noted that one effective method of increasing thermal capacitance is to include a material that undergoes a change of phase at a desirable temperature. Jet Impingement Technique: Jet impingement is an attractive cooling mechanism due to the capability of achieving high heat transfer rates. It was observed that Jet impingement has become a viable candidate for high-powered electronic and photonic thermal management solutions and numerous jet impingement studies have been aimed directly at electronics cooling. THEORYEvery electronic component depends on the passage of electric current to perform its obligation, and it becomes the potential site of heat generation. Current permeating active and passive components results in power dissipation and incremented temperatures. The amount of puissance dissipated by a contrivance is a function of geometry and the path from the contrivance to the heat sink. Power dissipated will be a function of the type of current that it receives for DC power dissipated,, and for AC is .Jet impingement is a cooling system of having a capability of having high heat transfer rates. The method of cooling is used in a wide range of applications in industry such as annealing of metals, cooling gas turbine blades, cooling in process of grinding and cooling of photovoltaic cells. Jet impingement has become a very important aspect of high-end electronic and photonic thermal management solutions; numerous jet impingements have been studied directly aimed at electronics cooling.With increase of power in electronic devices, processors, etc. that are requiring high capacity cooling methods to remove extra heat. A method of doing this is jet impingement of a gas or liquid in surface continuously. The mode of heat transfer has been studied and tested extensively for a lot of time and research is still going on. High speed jet impingement on a component surface creates a thin boundary layer, and thus a high heat transfer equation. With three common jet configurations: the free-surface jet, which uses dense liquid in a medium that is less dense; Submerged jet, allows fluid to impinge in the same medium fluid; and confined submerged jet, which is shown inFigure 1.Studies have been carried out that the spacing of jet-to-target has a large influence for heat transfer in a submerged jet than that for free-surface jets. Studies suggest very small change in stagnation and average heat transfer for ratio of H/D< 4, then a decrease of heat transfer occurs as H/d increases beyond this point.

EXPERIMENTThe experiment is conducted to observe the cooling patterns of the copper plate, which is used as a prototype for electronic equipment. The components used in the experiment can be described as follows: Components:

Fig 1. Experimental Setup Schematic

1. Test Plate: A copper plate of 0.01m diameter and thickness 5mm. The copper plate consists of holes drilled half way through to facilitate the placing of thermocouples. The surface is given appropriate finishing to give the best results. 2. Heater: A heating coil heater is used to heat the copper plate to the desired temperature. The capacity of the heater is about 1000W. 3. Thermocouples: Thermocouples are used at various points on the copper plate to measure the temperature at different points starting from the center of the plate. The thermocouples used are copper constantan, which were arranged at the required positions on the plate for the temperature readings. 4. Nozzle: The nozzle is the pipe through which the high velocity jet comes out. There are various types of nozzles, the plain type, swirling strip nozzle and the twisted nozzle. For this experiment we have used a plain nozzle for all the cases considered. The nozzle used had an internal diameter of 7mm and a thickness of 1mm. The length of the nozzle was developed such that by the time the jet reaches the end of the nozzle, it reaches fully developed flow. This was achieved by calculating the length to diameter ratio. 5. Pump: A pump was used to produce high velocity air. This pump is adjusted that it gives a constant mass flow rate of 2.77x10-4Kg/s. The temperature gained by the air in this process of increasing the velocity is neglected, as this temperature is extremely negligible compared to the temperatures attained by the pump at the time of heating. 6. Rotameter: A rotameter is used to make sure the mass flow rate is constant at different times by regular inspection and accordingly change the pump outlet if there is a deviation. 7. Stand: A very normal and fundamental stand was used to hold the pipe coming from the compressor to the nozzle in place and to change the heights and the stagnation point distances from the center. A basic table was used to place the heater and plate at appropriate height. The experiment setup is illustrated in the Figure 1.

ParametersThere are many parameters that have been considered for observation in the history of jet impingement. The various parameters that can be considered are listed below: H/D Ratio: The H/D Ratio is the ratio of height of the nozzle exit from the plate to the diameter of the nozzle. This is a very important parameter considered in the jet impingement technique. Nozzle type: The type of nozzle used is another variable. There are many types of nozzles like plane nozzle, nozzles with swirling strip and twisted nozzle. Angle of inclination: The angle at which the nozzle is impinged on to the jet. Stagnation point off set: The point where there is maximum heat transfer is called the stagnation point. This is usually the geometrical center when its a circular tube. However the cases considered in this experiment are listed as follows: 1. H/D = 5, R = 0 2. H/D = 5, R = 1.53. H/D = 3, R = 04. H/D = 3, R = 1.5. ProcedureThe experiment was conducted to using the components mentioned above to observe the cooling pattern of the copper plate. Ambient air is sucked into the pump to pressure high velocity jet. The flow rate of the jet is maneuvered using the outlet valve of the pump to make sure the mas flow rate of the jet coming out of the nozzle is maintained at 2.77x10-4kg/s. The high velocity airflows through the pipe enter the nozzle and come out of the nozzle tip. Because of the appropriate length selected, the flow will be fully developed by the time it reaches the tip. The copper plate is heated to 1750C and after it attains that temperature, the power or the heating is shut down. The jet from the nozzle is then impinged on to the heated copper plate and the after every fixed time interval, the temperature of various points on the plate are noted. The step-by-step procedure can be listed as follows: i. Initially the pump is switched on so that the pressure is built up and then it is run till the desired mass flow rate is obtained. ii. The heater is switched on and the plate is heated till it attains a temperature of 175oC.iii. Once uniform temperature of 175oC is attained on the test plate, we turn of the heater and impinge the jet on the test plate.iv. Readings are taken at different points on the test plate at regular time intervals using thermocouples.v. This procedure is repeated for the following casesa. Case 2 H/D=5, R=1.5b. Case 3 H/D=3, R=0c. Case 4 H/D=3, R=1.5vi. Graphs are plotted to show the variation of temperature with respect to time for various positions.

RESULTS AND DISCUSSION 1. H/D=3, R=0. It is observed from the Figure 2, that after the heater was switched off, the plate when impinged with jet begins to rapidly lose temperature at the stagnation point, which is at the center of the plate. Ten minutes later, the center obtains a temperature of 70 while the outer edge, i.e. the farthest point from the center has a temperature of 90 while the stagnation point cools off rapidly to a temperature of 70. The total time taken for the copper plate to attain a temperature of 45 using a plain nozzle with h/d=3 is 40mins.

2. H/D=5, R=0Figure 3 shows the cooling patters of various points on the heated plate. Considering the stagnation point, it lost temperature rapidly from 175 to 85 in the first ten minutes and then cools in a constant curve from 85 to 48 by the end of 40 minutes. From here it takes a slow pace to reach a temperature of 45 by the end of 60mins.

Fig 2 Cooling rate for H/D =3 and R=0

Fig 2 Cooling rate for H/D =3 and R=0

Fig 3 Cooling rate for H/D =5 and R=0

Fig 4 Cooling rate for H/D =3 and R=1.5 cm

3. H/D=3, R=1.5The nozzle position is shifted 1.5cm away from center towards the right side. Figure 4 shows the temperature variations from left side, indicated as negative distance and towards right, indicated as positive distances. It is observed from the graphs that unlike the above cases, the temperature of the points on the plate after 30mins lie in the same area and move together towards the end. It is also observed that by the end of 45mins, the temperature drops to about 50 and from there it slowly reduces to 45 by the end of 75mins.

4. H/D=5,R=1.5The pattern observed is similar to the above case. The data for this was obtained by CFD simulation. The results are shown in the following section.

Computational MethodCFD allows observation of flow properties without disturbing the flow itself, which is not always possible with conventional measuring instruments. Simulation was carried out for the following cases.

Table 1 Cases Carried Out For SimulationCaseH/D RatioOffset from Centre (in cm)

150

251.5

330

431.5

Flow domain was created in such a way that, the re-circulation that will created in a confined space for the current set was nullified by studying steady state solution. The geometries created were then meshed; in order to have a comparable result at the end of the simulation, the mesh characteristics for each of the cases had to be similar. The sizing option used was proximity and curvature. Proximity inserts a set number of cells between any two walls; curvature approximates a curved surface as a number of straight edges at a defined angle. In addition to the above, proximity and curvature sizing was selected as it enables mesh refinement at curves and gaps to resolve flow and easy control over number the elements over gaps, angle of curvature and other factors. The orthogonality achieved was 8.24x10-4 and maintained for all the flow domains.

Fig. 5 Mesh Created For the Flow DomainsThe solver was set to single precision and the scale value was changed from then default m to mm scale. The boundary conditions are as followsTable 2 Boundary Condition For Flow DomainsMass Flow Rate

Heat input while heating1000 W

The Enhanced wall treatmenttransport method was selected in order to solve the N-S equation. One the common turbulence models used to solve N-S equation is the K-epsilon model, although it just doesn't perform well in cases of large adverse pressure gradients [6]. It is atwo-equation model that means it includes two extra transport equations to represent the turbulent properties of the flow, thus, considering the history effects of convection and diffusion of turbulent energy. The first and the second transported variables are turbulent kinetic energy k and turbulent dissipation respectively. Where determines the scale of turbulence and k determines the energy in it. The first transported variable is turbulent kinetic energy,k. The second transported variable in this case is the turbulent dissipation,. It is the variable that determines the scale of the turbulence, whereas the first variable,k, determines the energy in the turbulence. Enhanced wall treatment is a near-wall modeling method that combines a two-layer model with enhanced wall functions. If the near-wall mesh is fine enough to be able to resolve the laminar sub layer (typically), then the enhanced wall treatment will be identical to the traditional two-layer zonal model [7]. The restriction faced is that, the near wall mesh be fine everywhere, this impose a large computational requirement. As the experiment involves the near wall region to be resolved into viscous sub layer, the integral part of the enhanced wall treatment is used.Fig.6 Cooling rate for different H/D =5 when impinged R=0 and R=1.5 cm

Table 3 Reference Values Computed from inlet for simulationArea (m2)3.85x10-5

Density (kg/m3)1.225

Enthalpy (j/kg)0

Length (mm)175

Temperature (K)298

Velocity (m/s)5.8733

Viscosity (kg/m-s)1.789e-05

Ratio Of Specific Heats1.4

The impinging air being delivered by the pump makes it difficult to measure the flow parameter, thus, hybrid initialization is used. Hybrid Initialization is a collection of recipes and boundary interpolation methods. When selected, a limited number of iterations are carried out to obtain values of floe parameters that are later used to carry out the main iteration. It solves the Laplace equation to produce a velocity field that conforms to complex domain geometries, and a pressure field, which smoothly connects high and low pressure values in the computational domain. All other variables will be patched based on domain averaged values or a predetermined recipe [7]. The computation was then started with time step size(s) of 1 sec with 1800 number of time steps. Cooling of electronic equipmentusually involves reducing the maximum temperature of the equipment, thus, vertexmaximum solution control wasselected and setto measure thestatic temperature along thediameter.Vertex Maximum Static Temperature

Vertex Maximum Static Temperature

Fig. 7 Cooling rate for H/D =3 when impinged at R=0 and R=1.5 cm

Vertex Maximum Static Temperature

Time Step

Fig. 8 Cooling rate for different H/D ratios at R=0

Vertex Maximum Static Temperature

Fig. 9 Cooling rate for different H/D ratios at R=1.5

CONCLUSIONS The experiments were performed to characterize the fluid flow and heat transfer behaviors produced by impinging air jets exiting a round nozzle. The effect of jet to plate spacing and distance of stagnation point from center were taken as the factors and were varied to obtain various results. The heat transfer rate by increasing the local heat transfer coefficient increases as the H/D ratio decreases i.e. jet spacing decreases owing to the reduction in the jet impinging area and vice versa. There is no significant change in temperature variation along y-axis, when the stagnation point is moved from center.RECOMMENDATIONS FOR FUTURE WORKThe cases studied in this experiment have a lot of scope for changes and improvements. Different angles of inclination of nozzle can be considered for experiment. Nozzle type can be changed to swirling nozzle, square nozzle, etc. and heat transfer rate for them can be investigated. The impingement jet configuration can be varied and its effects on film effectiveness can be investigated. Varying the mass flow rate and hence changing the Reynolds number, the cooling rates can be examined. As we increase H/D ratio beyond 5, Nusselt number variation can be investigated and thereby local heat transfer coefficient.

REFERENCES[1] D. K. Gomes, Experimental Investigation Of Air Cooling Systems For Electronic Equipment By Using Vortex Promoters, Rutgers University, MS Thesis, New Jersey, 2007. [2] Cooling techniques for electronic equipment Steinberg,D.S. New York, Wiley-Interscience, 1980. 387 p.[3] R. Kandasamy, X.-Q. Wang, A.S. Mujumdar, Application of phase change materials in thermal management of electronics, Applied Thermal Engineering in press, doi:10.1016/j.applthermaleng. 2006.12.013.[4] Theoretical Investigation of Sub-ambient On-Chip Microprocessor Cooling," Proceedings of the Tenth Intersociety Conference on Thermal and Thermo-mechanical Phenomena in Electronics Systems (ITHERM), 2006, San Diego, CA, 2006[5] N. ZUCKERMAN and N. LIOR,, Jet Impingement Heat Transfer: Physics, Correlations, and Numerical Modeling, Department of Mechanical Engineering and Applied Mechanics, The University of Pennsylvania,[6] Wilcox, David C (1998). "Turbulence Modeling for CFD". Second edition. Anaheim: DCW Industries, 1998. pp. 174.[7] ANSYS FLUENT Users Guide