When to Use a Heat Pipe and When to Use an Oscillating Heat Pipe?
Problem statement
Passive thermal management of opto-electronic, power-energy and communications systems is getting more difficult every day as devices are more powerful, smaller, and more densely packaged. Potential solutions include conventional, wick-based heat pipes and their newer, pressure-driven counterpart, Oscillating Heat Pipes (OHPs). Both transfer heat at orders of magnitude higher rates than solid materials; and do so without Size Weight and Power (SWaP) drawbacks of actively pumped cold plates.
But how to know which type of heat pipe is best for my application?
To answer that question, let’s first get acquainted with the “basics” of each.
Heat pipe basics
Heat pipes are a mature technology with decades of heritage in a range of industries. They acquire device-level heat fluxes and move 10s to 100s of watts up to a meter away. Heat pipe diameters are typically 3 to 12mm for linear heat transporters and can be made as flat, 2-D “vapor chamber” heat spreaders. Generally, the larger the diameter, the more power heat pipes can transfer; and, the thicker, more complex the internal wick-structure, the higher their heat flux limits. Each year billions of copper-water heat pipes are attached to microchips and connect to metal heat sinks in computers, servers and mobile devices. Alternative material-fluid pairs and highly-engineered vapor chambers are made for aero-defense and specialty applications but at lower volumes, higher prices. Ideal heat pipe working fluids have high surface tension and high latent heat of vaporization; as these properties diminish so do heat pipes’ operating limits which is why they are most commonly filled with water, and less often with ammonia or methanol.
OHP basics
OHPs have less than a decade of commercial heritage, mostly in the aero-defense industry. They are not mass produced but are growing in parallel with the rising demand for lightweight, power-dense opto-electronic and energy systems. This is because of OHPs’ unique capabilities vis-à-vis conventional heat pipes which include:
higher heat flux and total power limits
thinner, more structurally sound form factors
greater options in terms of 3D shapes, sizes and fluid-material pairs
better operability across variable gravity fields
OHPs today often are made as conformal “drop-in replacements” of solid metal structures but with 10 to 100x higher effective thermal conductivity (and 20-30% less mass). Aluminum is the most common material for OHPs, but copper, titanium and low coefficient of thermal expansion metals are also used. OHPs do not have internal wicks but instead are built from a meandering, 2- or 3-D internal micro-channel pattern partly filled with working fluid. A well-designed OHP can be made in nearly any shape for moving 10s to 1000s of watts meter-scale distances through cross-sections of 1-6 mm thick and 20-100 mm wide. Generally, the more channels, the more power (and flux) an OHP can handle; and, larger diameter channels have greater heat transport distances. Working fluid selection is critical to OHPs’ operating limits, but surface tension and latent heat are far less critical than in wick-based heat pipes. Refrigerants, butane, propylene, ammonia and even mixtures are common fluids – but water is difficult in OHPs due to its unique freeze-thaw, contraction-expansion cycle.
Figure 1 below is an illustrated comparison of 30mm wide x 100mm long heat sinks each attached to 15x15mm2 microchip with their approximate maximum heat transfer limits. From left to right: 5mm thick, two-heat pipe solution with approx. 60W power limit (30W per heat pipe); a 4mm thick vapor chamber with approx. 60W power limit (25W/cm2 flux); then a 2mm thick Al-Butane OHP with approx. 75W power limit; and finally a 2mm thick Al-Ammonia OHP with a 150W power limit. Each of these three options will produce an approx. 0.8 to 0.12 W/K thermal resistance depending on specific features (e.g., interface resistance, wick structures, or channel designs).
Figure 1. From left to right: Two-heat pipe heat sink (Cu-H2O), Vapor chamber heat sink (Cu-H20), OHP heat sink (Al-Butane) and OHP heat sink (Al-Ammonia).
And Figures 2, 3 and 4 are photographs of heat pipe, vapor chamber and OHP heat sinks made by ThermAvant Tech for its customers in years past, so that readers can further see the differences between heat pipes and oscillating heat pipes.
Figure 2. ThermAvant Tech heat pipe heat sink with bent Cu-H20 heat pipes and cut away of internal wick
Figure 3. ThermAvant Tech vapor chamber heat sink made with Cu-H20 and cut away of internal sintered wick
Figure 4. ThermAvant Tech OHP heat sink with illustrated 3D microchannel pattern and cut away of microchannels
