Boiling is a common mechanism for liquid-vapor phase transition and is widely exploited in power generation and refrigeration devices and systems. The efficacy of boiling heat transfer is characterized by two parameters: (a) heat transfer coefficient (HTC) or the thermal conductance; (b) the critical heat flux (CHF) limit that demarcates the transition from high HTC to very low HTC. While increasing the CHF and the HTC has significant impact on system-level energy efficiency, safety, and cost, their values for water and other heat transfer fluids have essentially remained unchanged for many decades. Here we report that the high surface tension forces offered by liquids in nanowire arrays made of Si and Cu can be exploited to increase both the CHF and the HTC by more than 100%.

Nanowires for enhanced boiling heat transfer.

We experimentally investigated surface roughness-augmented wettability on critical heat flux (CHF) during pool boiling with horizontally oriented surfaces. Microstructured surfaces with a wide range of well-defined surface roughness were fabricated, and a maximum CHF of ∼208 W/cm2 was achieved with a surface roughness of ∼6. An analytical force-balance model was extended to explain the CHF enhancement. The excellent agreement found between the model and experimental data supports the idea that roughness-amplified capillary forces are responsible for the CHF enhancement on structured surfaces. The insights gained from this work suggest design guidelines for new surface technologies with high heat removal capability.

Structured surfaces for enhanced pool boiling heat transfer

Pool-boiling CHF enhancement by modulated porous-layer coating: theory and experiment

Phase change through boiling is used in a variety of heat-transfer and chemical reaction applications. The state of the art in nucleate boiling has focused on increasing the density of bubble nucleation using porous structures and microchannels with characteristic sizes of tens of micrometers. Traditionally, it is thought that nanoscale surfaces will not improve boiling heat transfer, since the bubble nucleation process is not expected to be enhanced by such small cavities. In the experiments reported here, we observed unexpected enhancements in boiling performance for a nanostructured copper (Cu) surface formed by the deposition of Cu nanorods on a Cu substrate. Moreover, we observed striking differences in the dynamics of bubble nucleation and release from the Cu nanorods, including smaller bubble diameters, higher bubble release frequencies, and an approximately 30-fold increase in the density of active bubble nucleation sites. It appears that the ability of the Cu surface with nanorods to generate stable nucleation of bubbles at low superheated temperatures results from a synergistic coupling effect between the nanoscale gas cavities (or nanobubbles) formed within the nanorod interstices and micrometer-scale defects (voids) that form on the film surface during nanorod deposition. For such a coupled system, the interconnected nanoscale gas cavities stabilize (or feed) bubble nucleation at the microscale defect sites. This is distinct from conventional-scale boiling surfaces, since for the nanostructured surface the bubble nucleation stability is provided by features with orders-of-magnitude smaller scales than the cavity-mouth openings.

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Nanostructured copper interfaces for enhanced boiling.

Liquid–vapour phase change is a useful and efficient process to transfer energy in nature, as well as in numerous domestic and industrial applications. Relatively recent advances in altering surface chemistry, and in the formation of micro- and nanoscale features on surfaces, have led to exciting improvements in liquid–vapour phase-change performance and better understanding of the underlying science. In this Review, we present an overview of the surface, thermal and material science to illustrate how new materials and designs can improve boiling and condensation. There are many parallels between boiling and condensation, such as nucleation of a phase and its departure from a surface; however, the particular set of challenges associated with each phenomenon results in different material designs used in different manners. We also discuss alternative techniques, such as introducing heterogeneous surface chemistry or direct real-time manipulation of the phase-change process, which can offer further control of heat-transfer processes. Finally, long-term robustness is essential to ensure reliability and feasibility but remains a key challenge. Recent works in boiling and condensation have achieved unprecedented performance and revealed new mechanistic insights that will aid in material design. In this Review, we focus on nanoengineered materials, with emphasis on further improving the heat-transfer performance and long-term robustness.

Nanoengineered materials for liquid–vapour phase-change heat transfer


 The effect of structural modulation of porous-layer coating on the qCHF for horizontal surfaces is examined experimentally. The surface consists of a copper substrate coated with spherical copper particles which are diffusion sintered into modulated structures inside graphite molds. Results are reported for pentane boiling on four different surfaces; a plain uncoated surface, a surface with a porous-layer coating of uniform thickness, and two surfaces with different modulated porous-layer coatings. A two-fold increase in the critical heat flux (qCHF) is found for one of the modulated porous-layer coatings. A brief explanation of the hydrodynamic theory for the qCHF is then given in which it is postulated that the qCHF occurs when columnar, vapor-escape passages through the liquid become unstable and collapse. It is then hypothesized that the enhancement of the modulated coating results from surface-induced changes in the characteristic interfacial wavelengths that stabilize the liquid-vapor interfaces, delaying the collapse of these vapor escape paths, and ultimately increasing qCHF.

CHF enhancement by modulated porous-layer coating

The paper presents a model that makes a distinction between the contributions of the different mechanisms of heat transfer to an air bag fabric during deployment. It describes the experimental setup, designed for simulation and recording of the thermal response of permeable and coated air bag fabrics. Comparisons are made between the experimental results and numerical predictions.

Transient Heating of Air Bag Fabrics: Experiment and Modeling

Permeability and Transient Thermal Response of Airbag Fabrics

A numerical model is presented that is capable of isolating and quantifying the heat flux from the gas within an air bag to the air bag fabric due to internal surface convection during the inflator discharge period of an air bag deployment The model is also capable of predicting the volume averaged fabric temperatures during the air bag deployment period Implementation of the model into the Inflator Simulation Program is presented along with the simulation results for typical inflators The predicted effect of the heat loss from the bag gas to the fabric on the internal bag gas temperature and pressure and the resulting bulk fabric temperature as a function of fabric parameters and the inflator exit gas properties are presented for both permeable and impermeable air bag fabrics

S G Liter

Papers

Pool-boiling CHF enhancement by modulated porous-layer coating: theory and experiment

CHF enhancement by modulated porous-layer coating

Transient Heating of Air Bag Fabrics: Experiment and Modeling

Permeability and Transient Thermal Response of Airbag Fabrics

An experiment-based model of fabric heat transfer and its inclusion in air bag deployment simulations