1 Local temperature distribution Temperature contour of silicon material at X = 50 µm for p=50 kpa and heat flux 90 W/cm2 64 Figure 4.2 Temperature contour of silicon material at X = 50 µm for p=50 kpa. With the transition to CMOS circuit technologies in the 1990s, there was a brief respite; however, there has been a resurgence in demand for higher packing density and performance, and heat flux is again increasing at a challenging rate.
New Thermal Design Technology
Thermal management will play a crucial role for all types of electronic products in the coming decade. Design for performance, design for reliability, design for serviceability, design for expandability, design for minimal cost and design with minimal impact on the user.
Current Methods Used in Industry
- Internal Module Cooling
- External Module Cooling
- Immersion Cooling
- System-Level Cooling
- Air Cooling
- Hybrid Air-Water Cooling
- Liquid-Cooling Systems
- Refrigeration Cooled Systems
- Data Center Thermal Management
Approximately 50% of the heat transferred to the air in the table columns was transferred to the cooling water. Ultimately, hybrid air-liquid cooling offers the potential for a closed-loop, recirculating air cooling system with total heat rejection of the heat load absorbed from the air to the chilled water.
Heat Sink
While the power supply system of the multi-layer design is not significantly more complicated than the single-layer design, the current-wise temperature rise at the base of the surface is significantly reduced. At the same time, the pressure drop required for the multi-layer heat sink was significantly smaller than the single-layer design.
Micro channel Heat Sink
- Design Parameters
- Heat Sink Material
- The number of the fins
- Fin Shapes
- Other Parameters
The heat sink should be designed to give a smaller pressure drop than the static pressure of the fan. Designing a cooler with a smaller cross-sectional area than the flow area creates air bypass.
Objective of the Work
Cross-sectional area of flow:- The cross-sectional area of flow is known as long as the dimensions of the fan are known. By conveying the coolant to and from the heater through pipes, it is demonstrated that greater heat removal capabilities are achievable as air entrainment is prevented.
Organization of the Thesis
Literature Review
Introduction
With the advent of Computational Fluid Dynamics (CFD) in recent years, flow and heat transfer calculations have become entirely possible. Studies on microchannel flows in the last decade are classified into different topics.
Micro channel Concepts
The result shows that these fluid properties can significantly affect both flow heat transfer in the microchannel sink. In the last twenty-three years diligent research has been repeatedly carried out to better understand the single-phase and two-phase flow mechanisms of micro-channel chips. In an attempt to challenge current flow regulation for cooling electronic components, Vafai (15) compared the thermal performance of double-layer micro-channel heaters with conventional single-layer heatsinks.
The thermal performance of the stacked microchannel heat sink is affected by the channel aspect ratio, the thermal.
Analytical studies
Zhao (20) presented an analytical and numerical study on the heat transfer characteristics of forced convection over a microchannel heat sink. Finally, they performed two-dimensional numerical calculations for both constant heat flux and constant wall temperature conditions to check the accuracy of analytical solutions and to investigate the effect of different boundary conditions on the overall heat transfer. Zhao (25) this paper presents an analytical and numerical study on the heat transfer characteristics of forced convection over a microchannel heat sink.
Finally, two-dimensional numerical calculations are performed for both constant heat flux and wall temperature conditions to check the accuracy of the analytical solutions and to examine the effect of different boundary conditions on the overall heat transfer.
Numerical studies
The results show that the flow in the vicinity of the module is three-dimensional and exhibits flow separation and vortex formation, thus leading to a complex distribution of the local substrate heat transfer coefficient. They found that the effects of four parameters on thermal performance can all be explained by the principle of field synergy. The highest temperature is encountered on the hot surface of the cooler base immediately above the duct outlet.
Increasing the thermal conductivity of the solid substrate reduces the temperature on the heated base surface of the heatsink, especially near the duct outlet.
Experimental studies
The location of the module and the spacing of the modules relative to the flow were varied, and the effect of vortex generators on improving heat transfer was also studied. The effects of Reynolds number based on mean flow velocity and oscillation frequency on heat transfer enhancement are studied. When the pulsed frequency is in the locked regime, the heat transfer from the square cylinder is greatly increased.
The laminar heat transfer was found to be dependent on the aspect ratio and the ratio of hydraulic diameter to the center-to-center distance microchannel.
Introduction
In this chapter, the fundamental governing equations are continuity, momentum and energy equations derived from basic principles of heat and fluid flow. The equations are set to implement SIMPLE (Semi-implicit method for pressure-coupled equation) algorithm.
Governing Equations
- Momentum equation
Boundary conditions
Background of Theory
The Nusselt number is a dimensionless number that measures the increase in heat transfer from a surface that occurs in a real situation compared to the heat transferred if only conduction occurs. When the Rayleigh number is below the critical value for that fluid, heat transfer is mainly in the form of conduction; when it exceeds the critical value, the heat transfer is mainly in the form. ReDh= VD (3.25) Step 4 Hydraulic Diameter is calculated. 3.26) Step 5 Velocity and then mass flow rate are obtained.
COMPUTATIONAL FLUID DYNAMICS (CFD) & FLUENT
- FINITE VOLUME METHOD:-
- FINITE ELEMENT METHOD:-
- FINITE DIFFERENCE METHOD:-
This is the "classic" or standard approach most often used in commercial software and research codes. Where Q is the vector of conserved variables, F is the vector of fluxes (see Euler equations or Navier-Stokes equations), V is the cell volume and the cell area. Where Q is the vector of conserved variables, and F, G, and H are the fluxes in the x, y, and z directions, respectively.
This scheme assumes that the convective property at the interface is the average of the values of the adjacent interfaces.
GAMBIT AND FLUENT STEPS
- FLUENT STEPS:-
When this scheme is used, it produces an exact solution for any value of Peclet number and for any value of grid points. Control>Geometry>Face command button>Form face>Create face from Wireframe. To create face 'ABCD' select edges in an order (AB>BC>CD>DA) and click apply.
ANALYSIS OF GRID
SELECTION OF MODELS
CLOSURE
Introduction
Fluent performed a series of numerical calculations and the results are presented to demonstrate the effects of heat flux and mass flow rate on the temperature distribution in microchannel cooling sinks. Furthermore, to better compare the computational results obtained here with the experimental data available in the literature, the average total Nusselt number is defined and analyzed with respect to the Reynolds number variations. Since the thermophysical properties depend on the temperature, especially the viscosity of the fluid, the velocity and consequently the Reynolds numbers are different for the same pressure drop conditions.
As previously mentioned, the thermophysical properties of a fluid are based on the estimated total temperature of the fluid. For P = 50 kpa and 90 W/cm2, changing the reference temperature from 200 C to 32 0 C changes the average velocity from 1.12 to 1.38 m/s, and results in a corresponding change in Reynolds number from 99 to 146. here reference temperature 320C.
Température contour
Fig (4.6):- Temperature contour of liquid at outlet for Pressure difference of 15 kpa and heat flux of 90W/cm2. Fig (4.8): Temperature contour of liquid at outlet for Pressure difference of 50kpa and heat flux of 50W/cm2 at X=50µm. Fig (4.9): Temperature contour of liquid at outlet for Pressure difference of 50kpa and heat flux of 90W/cm2.
Fig (4.10):- Temperature contour of liquid at outlet for Pressure difference of 50kpa and heat flux of 150W/cm2.
Average heat transfer coefficient
Fig (4.12) Velocity contour at outlet of channel for pressure difference of 50 kpa and heat flux of 90 W/cm2. Fig (4.13) average heat transfer coefficient variation along the channel for for Pressure difference of 50kpa and heat flux of 90W/cm2.
Average Nusselt Number
The average heat transfer coefficient variation and the average Nusselt number variation for these three cases can be determined and are shown in (4.15). The nature of output results is comparable to conventional results. From these two figures, it can be concluded that the variations of the heat transfer coefficient and the Nusselt number along the flow direction are quite small for this type of microchannel heat sink after the thermal inlet lengths. It should be noted that since the grid size in the flow direction is relatively coarse, the local heat transfer is not as accurate or detailed as the case for the y and x direction.
Here we plot the average heat transfer coefficient variation along the channel length for different heat fluxes of the order of 50W/cm2, 90W/cm2 and 150W/cm2 in fig. (4.16).
Nusselt number variation for different heat flux
CONCLUSION
Here we compare the numerical results with other published results and experimental data available in the literature for Reynolds numbers less than 200 based on a hydraulic diameter of Dh = 86 µm. The influence of the geometrical parameters of the channel and the thermophysical properties of the fluid on the flow and heat transfer are investigated with a temperature-dependent thermophysical property. The results show that the variations in the way of determining the Nusselt number under different conditions.
This finding supports the concept of the micro-channel heat sink (MMC) where the flow length is greatly reduced to a fraction of the total length of the heatsink by using a design with multiple interconnected inlets and outlets.
Sources of Errors in CFD Calculations
Closure
For future work experimenting with MHEs, it is suggested to perform temperature measurements closer to the microchannel inlets and to measure temperatures at more locations in the solid surrounding the microchannels and near the surface where a complex boundary layer is formed to better understand how the heat flows through the entire MHE. Also, providing a higher wall temperature and/or a lower inlet temperature would reduce the experimental errors resulting from the temperature measurements. There is still a significant amount of work to be done experimenting with microchannels and MHEs, and the prospect of using MHEs in high-performance heat transfer applications looks more promising than ever.
BIBLIOGRAPHY
Judy, J., Maynes, D., and Webb, B.W., Frictional pressure drop characterization for fluid flows through microchannels, International Journal of Heat and Mass Transfer, 2002. Choi and Barron, Thermal Development Flow and Transfer of heat in rectangular microchannels of different aspect ratios, International Journal of Heat and Mass Transfer. Davies An experimental study of convective heat transfer in silicon microchannels with different surface conditions, Journal International of Heat and Mass Transfer, 2003.
Rodgers, Experimental study of convective heat transfer in silicon microchannels with different surface conditions, International Journal of Heat and Mass Transfer p.
