Porous Rib Structure Optimization in Ultra-Thin Vapor Chambers

Abstract

To manage increasing chip power densities in high-performance computing, vapor chambers have emerged as a leading passive cooling solution, but ultra-thin designs require support columns that obstruct vapor flow and compromise thermal uniformity. This thesis addresses how the number and arrangement of support ribs affect the coupled thermal-hydraulic behaviour. The primary goal was to develop a validated three-dimensional transient numerical model and use it to investigate the effect of rib number across multiple heat flux levels. The study is framed as a preliminary exploration of the design space rather than a formal optimization procedure, focusing specifically on the effect of rib number and arrangement while keeping rib thickness, cross-sectional shape, and spacing fixed. First, a no-rib baseline was simulated and confirmed to lose two-phase operation: lacking the parallel conduction paths and the auxiliary capillary return that the ribs provide, the evaporator superheats to roughly 197 °C and boils off the local liquid, yet the vapor never circulates or condenses back, so the chamber dries out. This verifies the necessity of support structures and validates the model's ability to capture this failure mode. Subsequently, a 12-rib, a 14-rib, and a symmetric 16-rib (4×4 matrix) configuration — the 12- and 14-rib layouts obtained by selectively removing non-critical ribs from the 16-rib baseline — were compared at heat fluxes of 80, 400, and 800 kW/m2.
A key finding is that increasing the rib count does lower the peak temperature — at 400 kW/m2 the peak wick temperature falls from approximately 164 °C for the 12-rib layout to 94 °C for 14 ribs and 44 °C for 16 ribs — but the vapor volume fraction reveals that this reduction is driven by a change of operating regime rather than by improved two-phase performance. At the same heat flux the vapor volume fraction drops from full saturation (VF ≈ 1.0, VC-12) through partial saturation (VF ≈ 0.88, VC-14) to essentially zero (VF ≈ 0.005, VC-16): the denser the rib array, the more the chamber reverts from phase-change-dominated operation towards pure solid conduction. The low temperature of the 16-rib layout therefore reflects a near-solid metallic spreader rather than an efficiently working vapor chamber. Selecting the rib count is thus a trade-off between mechanical support and preserving an active vapor phase at the design heat flux. For the 12-rib configuration, which remains an active vapor chamber across the tested range, a tentative operating window of approximately 200–600 kW/m2 is suggested on the basis of the observed trends.

Popular Abstract

Every time you record a 4K video or play a demanding game, the chip in your phone pours out a remarkable amount of heat from a space smaller than a fingernail — in terms of heat per unit area, not far off a kitchen hot plate. If that heat is not carried away quickly, the phone throttles its own speed or shuts down. Hidden inside most flagship phones and thin laptops is an elegant answer to this problem: a vapor chamber, a sealed metal pocket thinner than a credit card that behaves like a miniature weather system.
Inside that pocket, a tiny amount of water sits in a sponge-like lining. Over the hot chip it boils into vapor, soaking up a large amount of heat in the process. The vapor spreads to the cooler parts of the chamber, condenses back into liquid, and is then drawn back to the hot spot through the sponge to start again — all with no fan, no pump, and no moving parts. This quiet cycle of evaporation and condensation lets the chamber spread heat far more effectively than a solid block of copper of the same size.
The catch with going ultra-thin
There is a snag. To fit inside ever-slimmer devices, these chambers are squeezed down to a fraction of a millimetre. At that thinness the chamber would simply cave in under its own internal vacuum, so engineers add small internal pillars, or “ribs”, to hold the roof up. But those same ribs sit right in the path of the flowing vapor and get in its way. So how many ribs should a designer use? Too few and the chamber collapses; too many and the vapor can no longer move. This thesis set out to answer that question using detailed computer simulations of the heat and fluid flow inside the chamber.
A cooler reading that hides a broken cooler
We compared chambers with 12, 14 and 16 ribs, plus a control case with none at all. At first the results looked simple: the more ribs we added, the lower the peak temperature became. The 16-rib design ran coolest, which would normally be declared the winner. But a closer look revealed a trap. The dense forest of metal ribs had quietly turned the chamber into little more than a solid metal plate. Heat was now travelling straight through the metal by ordinary conduction, and the clever boiling-and-condensing cycle — the whole reason a vapor chamber exists — had switched off almost entirely. The low temperature was real, but it came from disabling the very mechanism the device is built around.
The leaner 12-rib design, with a wider open channel for the vapor, ran hotter on paper — but it was the only layout that kept the vapor cycle genuinely alive across the whole range of power levels we tested. In other words, a slightly warmer chamber that is actually working can be the better engineering choice than a cooler one that has stopped doing its job. The control case with no ribs confirmed the danger from the other direction: it overheated dramatically and dried out completely.
Why it matters
The main message is that designing these chambers is a balancing act, not a race to add as much support as possible. Choosing the number of ribs means trading off mechanical strength, the evenness of the cooling, and — most importantly — keeping the vapor cycle switched on. Just as crucially, the work is a reminder that a single comforting number, like a low temperature, can hide what is really going on inside. As phones, laptops and data-centre chips keep getting more powerful in ever-tighter spaces, getting this balance right is part of what keeps the device in your hand fast, reliable, and cool to the touch.

• author: Wu, Siqi and Gao, Chenzi
• supervisor: Himani Garg
• organization: Department of Energy Sciences
• course: MVKM05 20261
• year: 2026
• type: H2 - Master's Degree (Two Years)
• subject: Technology and Engineering
• keywords: MSc, Vapor chamber, Support ribs, Thermal management, Phase-change heat transfer, Computational fluid dynamics (CFD), Heat flux, Porous wick, Thermal resistance
• report number: ISRN: LUTMDN/TMHP-26/5694-SE
• ISSN: 0282-1990
• language: English
• id: 9237934
• date added to LUP: 2026-06-16 10:32:35
• date last changed: 2026-06-16 10:32:35