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Journal ArticleDOI

Working-fluid selection for minimized thermal resistance in ultra-thin vapor chambers

TL;DR: In this paper, a resistance-network-based model is used to develop a simple analytical relationship for the vapor chamber thermal resistance as a function of the working fluid properties, operating power, and geometry.
About: This article is published in International Journal of Heat and Mass Transfer.The article was published on 2017-03-01 and is currently open access. It has received 38 citations till now. The article focuses on the topics: Working fluid & Compressed fluid.

Summary (2 min read)

1. Introduction

  • Portable electronic device platforms such as smartphones and tablets are trending toward thinner, more compact designs with greater embedded functionality (which in turn leads to more waste heat generation from active components).
  • Vapor is generated at the evaporator section.
  • One conventional ‘figure of merit’ used for guiding the choice of working fluid prioritizes maximizing the operating power.
  • The thermal resistance of vapor chambers becomes dominated by the temperature gradient in the vapor core as the thickness is reduced.

2. Model

  • A working fluid should be chosen to yield the best possible thermal performance, typically characterized in terms of the effective thermal resistance of the vapor chamber.
  • A physics-based transport model for vapor chamber operation which predicts the effective thermal resistance is hence required to inform working fluid selection.
  • The entire opposing face acts as the condenser.
  • (1) As a design premise, the wick thickness should be minimized to enable the largest vapor core thickness possible; a required minimum wick thickness is computed based on the capillary limit at power Q for each fluid.
  • The same fluid selection approach presented here could be applied using alternative, high-fidelity model frameworks [14, 15].

2.1 Design for minimized wick thickness

  • For a vapor chamber to operate, the capillary pressure driving the fluid flow must be larger than the pressure drop.
  • To design for the minimum required wick thickness, the capillary pressure is equated to the pressure drop in the wick (i.e., capillary limit at this minimum thickness).
  • The pressure drop in the vapor core, although larger than conventional ‘thick’ vapor chambers, is still typically significantly less than the pressure drop in the wick for ultra-thin vapor chambers, and therefore is not considered.
  • The pressure drop in the wick is computed using Darcy’s law for one-dimensional radial flow.
  • The authors assume that the rate of condensation is uniform across the entire interface (constant mass flux across the interface) to obtain a simplified analytical expression for the mass flow rate: 2 wick fg Q r m r h R Hence, the mass flow rate is expressed as: .

2.2 Expression for vapor core effective conductance as a function of Mv

  • The temperature gradient in the vapor core is due to the saturation pressure gradient.
  • The pressure gradient is computed using the steady-state fluid momentum transfer equation (cylindrical coordinates) in the radial direction.
  • The assumption is valid when the vapor core resistance is larger than all other primary resistances (viz., the diffusive thermal resistance in the wick and the solid wall and the resistance due to phase change).
  • The model developed above indicates that the vapor core effective conductance (Eq. (19)) increases with an increase in either of the conventional figures of merit that contain both liquid properties (Ml) and vapor properties (Mv).

3.1 Effect of operating power and working thickness on the choice of working fluid

  • The operating power has a significant effect on fluid choice.
  • Thus, pentane is heavily penalized for its low Ml, and acetone is the best choice.
  • At this power, the contour plot includes a vertical line marked Ml,min.
  • The effect of power and working thickness on fluid choice is apparent in this map .
  • Thus, the vapor core effective conductance for all the fluids is changed by the same factor related to operating power, and the relative performance between different fluids is unchanged on these loci.

3.2 Effect of operating temperature on the choice of working fluid

  • The temperature-dependence of the thermophysical fluid properties affects the choice of working fluid that would yield the best performance.
  • For computing fluid properties, the operating temperature can be defined as the area-weighted average temperature on the surface of the condenser because the temperature difference across the thickness of the vapor chamber is minimal.
  • The appearance of the map changes based on the temperature-dependent thermophysical properties of each fluid.
  • It is critical to note that the walls of the vapor chamber must support the pressure difference between the vapor core and ambient, and mechanical design considerations may exclude some working fluids.
  • In the maps shown in Figure 5, fluids which have a vapor pressure higher than an arbitrary limit of 3 atm are shown crosshatched.

4. Conclusion

  • This work investigated the effects of the thermophysical properties of working fluids on the performance of ultra-thin vapor chambers.
  • At these form factors, the vapor chamber thermal resistance is dominated by the fluid flow in the vapor core.
  • A methodology for selecting between working fluids for a given set of ultra-thin vapor chamber geometric and operating parameters was developed.
  • 2) With decreasing vapor chamber thickness, the preference changes from a fluid with high Mv to one with high Ml; at the lowest thicknesses, a high Ml becomes a requirement so that the wick does not occupy the entire thickness available; and 3).
  • The unique temperature-dependence of thermophysical properties for each fluid govern fluid selection; caution must be exercised to ensure a reasonable vapor pressure at which the structural integrity of the vapor chamber is not compromised.

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Citations
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Journal ArticleDOI
TL;DR: This research presents a comprehensive review of the state-of-the-art for both rigid and flexible ultra-thin heat pipe technology for flexible thermal management solutions for foldable and wearable devices.

36 citations

Journal ArticleDOI
TL;DR: In this paper, the authors investigated the thermal resistance of ten vapor chambers with different configuration structures with different working fluid fill ratios and found that the optimal fill ratio to obtain the minimum thermal resistance depends on the operating power, geometries and material types of heat sinks inside the vapor chamber.

31 citations

Journal ArticleDOI
TL;DR: In this paper, the meniscus location, capillary pressure, effective thermal conductivity, and permeability are also predicted for heterogeneous, periodic sintered copper-particle (including bimodal particle size) unit cells.

30 citations


Cites methods from "Working-fluid selection for minimiz..."

  • ...The fluid property based figure of merit for uniform-wick vapor chamber has been introduced for both liquid and vapor phases and used to find optimal working fluid in [35]....

    [...]

Journal ArticleDOI
TL;DR: In this paper, it is shown that a heat pipe can sustain heat loads higher than the steady-state capillary limit for brief periods of time without experiencing dryout, even after the heat load is reduced back to a level lower than the capillary threshold.

29 citations

Journal ArticleDOI
TL;DR: In this article, a high-performance multiple mesh wick structure, fabricated by oxidation treatment and sintering, was proposed for enhancing the thermal performance of ultra-thin heat pipes (UFHPs).

29 citations

References
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02 Apr 2007

3,356 citations


"Working-fluid selection for minimiz..." refers methods in this paper

  • ...The thermophysical properties of the fluids are computed using the REFPROP database [17]....

    [...]

Journal ArticleDOI
TL;DR: In this article, the authors compared three monoporous and 19 biporous wicks and found that the monoporous wick can reach higher critical heat flux (CHF) than thin biporous Wicks because they developed evaporating menisci not only on top surface of wick but also inside the wick.

169 citations


"Working-fluid selection for minimiz..." refers background in this paper

  • ...The design of such vapor chambers typically focuses on the evaporator wick, and aims to reduce thermal resistance in the evaporative [5, 6] or boiling regimes [7, 8]....

    [...]

Journal ArticleDOI
TL;DR: In this article, a transient, three-dimensional model for thermal transport in heat pipes and vapor chambers is developed, where the Navier-Stokes equations along with the energy equation are solved numerically for the liquid and vapor flows.

117 citations

Journal ArticleDOI
Ravi Prasher1

110 citations


"Working-fluid selection for minimiz..." refers methods in this paper

  • ...To develop a standard practice for working fluid selection, we base the selection process in this work on a conventional thermal resistance network modeling approach [13]....

    [...]

Frequently Asked Questions (1)
Q1. What have the authors contributed in "Working-fluid selection for minimized thermal resistance in ultra-thin vapor chambers" ?

A working fluid is sought in this case that provides the minimal thermal resistance while ensuring a capillary limit is not reached at the target operating power.