Shear Drive Boundary Layer Reduction for Heat Rejection
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Solution Overview
Problem
Viscous fan drives face challenges in heat rejection due to the generation of 'slip heat' in a small volume of working fluid with a limited wetted surface, leading to high thermal gradients and inefficient heat transfer, particularly at high torque conditions where turbulent flow is not possible due to the high viscosity of the working fluid.
Innovation Solution
The apparatus incorporates flow altering structures on the outer housing assembly, such as cavities and ribs, to reduce the thickness of the boundary layer and induce mixing in the shear zone, promoting a more uniform temperature gradient and enhancing heat transfer in the laminar flow conditions characteristic of high viscosity, non-Newtonian working fluids.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If cooling fins are added to the outer housing assembly to increase heat rejection surface area, then the ability to reject heat to the atmosphere is improved, but the heat transfer from the working fluid to the outer housing assembly remains inefficient due to the thick boundary layer
Solution Approach 1:
The patent applies local quality by creating flow altering structures (protrusions and recesses) at specific locations within the working cavity where boundary layer thickness is problematic. These localized structures modify the flow characteristics in specific regions rather than uniformly across the entire cavity, thinning the boundary layer where it matters most while maintaining other desirable flow properties elsewhere.
Solution Approach 2:
The flow altering structures act as intermediaries between the working fluid and the outer housing assembly. These structures (protrusions extending into the fluid and recesses in the housing) mediate the heat transfer process by disrupting the thick boundary layer that normally insulates the fluid from the housing, thereby enhancing thermal coupling without directly changing the housing geometry.
2Loss of energy
If the working fluid volume is increased to improve heat transfer capacity, then more fluid is available for heat rejection, but the device size and complexity increase
Solution Approach 1:
The patent applies parameter changes by modifying the flow characteristics (Reynolds number, boundary layer thickness) through geometric alterations rather than changing the volume of working fluid. The protrusions and recesses change the flow regime from thick-boundary-layer laminar flow to thinner-boundary-layer flow, improving heat transfer efficiency without requiring additional fluid volume or increasing device size.
3Force
If the shear gap between the disk and outer housing assembly is reduced to increase torque transmission, then higher torque is achieved, but the boundary layer thickness increases and heat transfer deteriorates
Solution Approach 1:
The patent applies segmentation by dividing the continuous shear gap into multiple zones using protrusions and recesses. This segmentation creates regions of different gap thicknesses within the overall shear gap, allowing the system to maintain a small average gap for high torque while creating localized regions with thinner boundary layers for improved heat transfer.
Solution Approach 2:
The patent addresses the two-dimensional trade-off between gap size and boundary layer thickness by introducing a third dimension - radial protrusions and recesses that extend into the shear gap. This dimensional addition allows independent control of torque transmission (determined by average gap size) and heat transfer (determined by local boundary layer thickness at the housing surface).
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The configuration significantly reduces the boundary layer thickness, improving heat rejection from the working fluid to the outer housing assembly, allowing for increased torque transmission while limiting the working fluid temperature, thus addressing the inefficiencies in heat transfer and thermal management.
Implementation Method 1
reduce a thickness of a boundary layer of the working fluid adjacent the at least one of the first annular wall and the circumferentially extending wall
Implementation Method 2
completely laminar shear layers are set up that do not effectively transport thermal energy from layer to layer
Implementation Method 3
increase heat rejection from a working fluid
Implementation Method 4
The disk and the outer housing assembly cooperate to form a flow path that is configured to generate shear forces in the working fluid that in turn creates torque
Implementation Method 5
increase the ability of these devices to reject heat to the atmosphere via conduction, convection and radiation
Implementation Method 6
increase the ability of these devices to reject heat to the atmosphere via conduction, convection and radiation
Implementation Method 7
increase the ability of these devices to reject heat to the atmosphere via conduction, convection and radiation
Data Source
AI summary
A device that employs shear forces to transmit energy includes an outer housing assembly, a disk, and a reservoir with a working fluid. The disk is received in and rotatable relative to the outer housing assembly. A working cavity is formed between a rotor portion of the disk and the outer housing assembly into which the working fluid is received to create shear forces. A plurality of flow altering structures are disposed on the outer housing assembly and are configured to reduce a thickness of a boundary layer of the working fluid in the working cavity in areas that are local to the flow altering structures.