Potential for improvement in high heat flux HyperVapotron element performance using nanofluids

Abstract

HyperVapotron (HV) elements have been used extensively as high heat flux beam stopping components in nuclear fusion research facilities. These water-cooled heat exchangers use a boiling heat transfer mechanism and so are inherently limited by their critical heat flux (CHF). The use of a nanofluid as the coolant, instead of water, promises to enhance the heat transfer performance of the HV and increase the CHF by a factor of 2 or 3, which would lead to a step-change improvement in the power handling capability. This paper reports on computational and experimental analyses which have indicated mechanisms for the enhanced thermal performance of nanofluids. A molecular dynamics simulation code has been developed which has identified heat transfer augmentation mechanisms that depart from classical thermodynamics associated with the presence of nanoparticles. In addition, an experiment has been conducted which uses particle image velocimetry to measure the flow field in a full-scale HV. Past studies have yielded qualitative experimental results, but the measurements reported here provide quantitative data to aid the understanding of the initial flow field inside the HV (i.e., before a heat flux is applied). Further, the experiment is conducted using both water and Al2O3–water nanofluid as the flow medium. Thus, these velocity measurements offer a first indication for potentially enhanced heat transfer in HV devices when nanofluids are used as the coolant. The improved understanding of the HV flow regime and the cooling advantage of nanofluids could assist the design of advanced high heat flux components for future fusion machines.

Revealing the complex conduction heat transfer mechanism of nanofluids

“…Although the size parametric study was limited to smaller nanoparticle sizes due to the limited available computational time, it is expected that the phenomenon will reach a maximum when the inertial effects of the nanoparticle will overcome the observed diffusive phenomena. This mechanism is in agreement with the experimental observations linking nanoparticle size and thermal conductivity of a nanofluid [2][3][4][5][6][7][8][9][10][11][12], and it might hence explain the heat transfer enhancement exhibited by nanofluids-excluding any quantum effects.…”

“…Nanofluids exhibit an enhancement over their base fluids of the order of 5-9 %, 10-14 %, 40-44 % and 100-200 %, respectively, for the purely conductive, mixed convective/ conductive, pool boiling and critical heat flux (CHF) heat transfer modes. The physical mechanisms that give rise to this enhancement are not yet understood, and hence, the design flexibility offered by the increased degrees of freedom of the mixture parameters (nanoparticle and carrier combination materials, flow application type and state, nanoparticle size, surface treatment, shape and concentration as well as temperature range of application) for maximising their performance at specific applications cannot be defined [2][3][4][5][6][7][8][9][10][11][12].…”

Assessing the flow characteristics of nanofluids during turbulent natural convection

“…PIV has been already employed to investigate flows of dilute nanofluids (semi-transparent fluids) inside various systems, such as a HyperVapotron [34] and a pool boiling apparatus [35]. The PIV technique relies on the use of a laser source to illuminate twice, with a fixed time interval, micron-sized tracer (seeding) particles dispersed in the flow, on planes defined by a thin laser sheet.…”

Isothermal velocity measurements in two HyperVapotron geometries using Particle Image Velocimetry (PIV)

“…Both geometries are evaluated under three operating conditions, which are defined by the free stream speed of the flow inside the devices, with one of them being the typical operating condition on both fusion experiments. The current paper builds on the preliminary results published in [27] by presenting a wide range of experimental results for a range of flow regimes. Quantitative temporal and spatial flow analysis is applied to the instantaneous measurements, including Proper Orthogonal Decomposition (POD) of the instantaneous flow distribution and image recognition of the instantaneous vortical centroid structure present inside the grooves of the HV.…”

“…The hydrodynamic flow structures evolving in the device have not been studied in detail experimentally and hence the physics of heat transfer, which are strongly related to the coolant flows, are not yet fully understood. This paper is a significant expansion of our earlier work [27] and addresses the temporal behaviour of the isothermal flow field inside the grooves of a HV, which is important for the single phase heat transfer mode that initiates the 'Vapotron' effect during hot operation. This should enable a better understanding of both the design aspects and operation regimes of the device, as well as a better understanding of the initiation of the 'Vapotron' effect by analyzing the single phase heat transfer in the device before the vaporisation and ejection processes take place.…”