Geoffrey F. Hewitt’s research while affiliated with Imperial College London and other places

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Publications (108)


Figure 1: Gas flow in tubes/pores without (left) and with (right) slip.
Figure 2: At low pressures an artificial increase in permeability can be observed.
Table 2 .
Figure 3: Rate of oxygen diffusion through sandstone sample S3 measured experimentally for steady state diffusion interaction: increase of O 2 concentration in N 2 purge stream, as well as experimental transient diffusion of O 2 as fraction of the final equilibrium value.
Figure 5: a) schematic (a) and photograph (b) of the permeability cell set-up (not to scale)

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An integrated method for measuring gas permeability and diffusivity of porous solids
  • Article
  • Full-text available

April 2020

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529 Reads

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1 Citation

Chemical Engineering Science X

Shu San Manley

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Geoffrey F. Hewitt

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An integrated gaseous transport apparatus was designed not only to measure permeability (by the pressure rise technique) but also to study both steady state and (importantly) transient diffusion of oxygen in nitrogen in the pores of the porous material. The apparatus allows for the characterisation of porous media with a wide range of permeabilities and yielded accurate values of the viscous permeability and the slip flow coefficient. Experiments were also carried out in the non-Darcy flow regime. The diffusion measurements provide information on the ratio of the diffusion coefficient in the media to that in free space; this ratio is also a specific property of porous media. Combining the steady state and transient diffusion measurements, it was possible to deduce the effective porosity and the pore length. Berea sandstones with a range of porosities were analysed using both techniques and the results of the measurements are presented.

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An integrated method for measuring gas permeability and diffusivity of porous solids

April 2020

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68 Reads

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8 Citations

Chemical Engineering Science

An integrated gaseous transport apparatus was designed not only to measure permeability (by the pressure rise technique) but also to study both steady state and (importantly) transient diffusion of oxygen in nitrogen in the pores of the porous material. The apparatus allows for the characterisation of porous media with a wide range of permeabilities and yielded accurate values of the viscous permeability and the slip flow coefficient. Experiments were also carried out in the non-Darcy flow regime. The diffusion measurements provide information on the ratio of the diffusion coefficient in the media to that in free space; this ratio is also a specific property of porous media. Combining the steady state and transient diffusion measurements, it was possible to deduce the effective porosity and the pore length. Berea sandstones with a range of porosities were analysed using both techniques and the results of the measurements are presented.


Modelling Strategies and Two-Phase Flow Models

January 2018

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102 Reads

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6 Citations

The general methods of solution of thermal-hydraulic problems are recalled first and then we show how these are complicated by the presence of multiphase flows, before entering into the descriptions of the various approaches commonly used. The special features of multiphase or two-phase flows are pointed out, in particular the existence of a number of flow regimes. The various two-phase flow and boiling heat transfer variables of interest, such as the pressure gradient, the void fraction, the heat transfer coefficient, etc., will depend on the particular flow regime. In principle, one should model each flow regime separately; when flow-regime-specific models are used, one can “mechanistically” take into consideration the particularities of each regime. The alternative approach often used is to largely ignore the flow regimes and derive methods (most often empirical correlations) covering all flow regimes continuously. The complete formulation of the two-phase-flow problem, which would have required the description of the evolution in time of the fields (pressure, velocity, temperature, etc.) for each phase, together with a prediction of the geometry of the interfaces, is generally impractical. The often chaotic flow fields must be treated in terms of statistical, average properties. There are two general approaches, the two-fluid, or more generally the multi-fluid approach and the mixture formulation. A simple presentation of the two-fluid approach is given. The basis of the method is to write conservation equations for each phase and to include in these equations terms which represent the interaction between the phases. The closure laws required to complete this formulation are listed and examples of implementation difficulties are given. The phase conservation equations may be summed up to yield mixture conservation equations, a particular case is the homogeneous flow model. The relatively new developments that rely on computational fluid mechanics methods to analyse and simulate multi- and two-phase flows are introduced to the reader.


Introduction to Multiphase Flow: Basic Concepts, Applications and Modelling

January 2018

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526 Reads

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37 Citations

This book is the maiden volume in a new series devoted to lectures delivered through the annual seminars “Short Courses on Multiphase Flow,” held primarily at ETH Zurich continuously since 1984. The Zurich short courses, presented by prominent specialists in the various topics covered, have attracted a very large number of participants. This series presents fully updated and when necessary re-grouped lectures in a number of topical volumes. The collection aims at giving a condensed, critical and up-to-date view of basic knowledge on multiphase flows in relation to systems and phenomena encountered in industrial applications. The present volume covers the background of Multiphase Flows (MPF) that introduces the reader to the particular nature and complexity of multiphase flows and to basic but critical aspects of MPFs including concepts and the definition of the quantities of interest, an introduction to modelling strategies for MPFs, flow regimes, flow regime maps and transition criteria. It also deals with the ubiquitous needs of the multiphase-flow modeller, namely pressure drop and phase distribution, i.e., the void fraction and the topology of the phases that determines the flow regimes.


Flow Regimes

January 2018

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85 Reads

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5 Citations

The chapter begins by describing the various two-phase flow regimes observed in pipes. Flow regime or pattern maps are used to identify the flow regime in terms of flow variables. Selected empirical flow regime maps are presented and the ways in which these may be generalized is discussed. A discussion of the analytical description of some important individual flow-pattern transitions follows. Flow regime maps can be analytically put together by using a set of transition criteria between flow regimes. The generation of full sets of criteria that can be used to build flow-pattern maps is described. Most commonly used flow regime maps for horizontal, vertical and inclined pipes are discussed at some length. This chapter is very extensive and long as it goes well beyond the description of the flow regimes and introduces the mechanistic methods used to define the transitions between regimes and their assembly into analytical flow regime maps.


Figure 1. Schematic of DRACS, showing the molten salt/air heat exchanger NDHX.
Figure 2. Schematic of the experimental apparatus used to conduct transient freezing measurements.
Figure 3. Schematic of the test section used to conduct transient freezing measurements.
Figure 4. Measured and calculated thickness of the solidified layer for an inlet water temperature of 8 °C and a Reynolds number of 20.
Figure 5. Growth rates of the solidified layer (experiments and simulations) for different experimental conditions
Tackling Coolant Freezing in Generation-IV Molten Salt Reactors

July 2017

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278 Reads

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1 Citation

In this study we describe an experimental system designed to simulate the conditions of transient freezing which can occur in abnormal behaviour of molten salt reactors (MSRs). Freezing of coolant is indeed one of the main technical challenges preventing the deployment of MSR. First a novel experimental technique is presented by which it is possible to accurately track the growth of the solidified layer of fluid near a cold surface in an internal flow of liquid. This scenario simulates the possible solidification of a molten salt coolant over a cold wall inside the piping system of the MSR. Specifically, we conducted measurements using water as a simulant for the molten salt, and liquid nitrogen to achieve high heat removal rate at the wall. Particle image velocimetry and planar induced fluorescence were used as diagnostic techniques to track the growth of the solid layer. In addition this study describes a thermo-hydraulic model which has been used to characterise transient freezing in internal flow and compares the said model with the experiments. The numerical simulations were shown to be able to capture qualitatively and quantitatively all the essential processes involved in internal flow transient freezing. Accurate numerical predictive tools such the one presented in this work are essential in simulating the behaviour of MSR under accident conditions.


Heat transfer in production and decay regions of grid-generated turbulence

June 2017

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119 Reads

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23 Citations

International Journal of Heat and Mass Transfer

Heat transfer measurements around the centreline circumference of a cylinder in crossflow are performed in a wind tunnel. The cylinder is placed at several stations downstream of three turbulence-generating grids with different geometries and different blockage ratios : a regular grid (RG60) with , a fractal-square grid (FSG17) with and a single-square grid (SSG) with . Measurements are performed at 20 stations for 3 nominal Reynolds numbers (based on the diameter D of the cylinder) . Hot-wire measurements are performed along the centreline, without the cylinder in place, to characterise the flow downstream of the grids. The extent of the turbulence production region, where the turbulence intensity Tu increases with the streamwise distance x from the grid, is higher for SSG and more so for FSG17 than for RG60. The angular profiles of the Nusselt number Nu are measured in the production regions of these two grids and are compared to those obtained in the decay regions, where Tu decreases with x. This comparison is made at locations with approximately same Tu. It is found that, for SSG, on the front of the cylinder (boundary layer region) is lower in the production region than in the decay region. This is explained by the presence of clear and intense vortex shedding in the production region of SSG which reduces the turbulent fluctuations which are “effective” in enhancing the heat transfer across a laminar boundary layer. For higher , the values of on the front of the cylinder are higher in the production region of FSG17 than in that of SSG, despite Tu being higher for SSG. This is consistent with a lower intermittency of the flow for FSG17 caused by the presence of the fractal geometrical iterations. The recovery of Nu on the back of the cylinder (wake region) is appreciably higher in the production region than in the decay region for both FSG17 and for SSG. This can be due to the lower integral length scale ratio in the production region and suggests, for the same , a reduction of the vortex formation length downstream of the cylinder, possibly promoted by the interaction between the wakes of the bars of the grid and the wake of the cylinder. At a large distance from the grids, the heat transfer enhancement is higher and it is more efficient for FSG17 and for SSG than for RG60. For high values of x in the turbulence decay region of the grids, the values of (circumferential average of Nu) are similar for FSG17 and for SSG and they are both appreciably higher than for RG60. This happens despite both FSG17 and SSG having a lower blockage ratio than RG60. The use of FSG17 has the practical advantage of combining high heat transfer rates on the cylinder with a weak vortex shedding from the grid.


Numerical study of three-dimensional droplet impact on a flowing liquid film in annular two-phase flow

April 2017

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196 Reads

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43 Citations

Chemical Engineering Science

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Geoffrey F. Hewitt

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[...]

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Annular flow with liquid entrainment occurs in a wide variety of two-phase flow system. A novel control volume finite element method with adaptive unstructured meshes is employed here to study three-dimensional droplet deposition process in annular two-phase flow. The numerical framework consists of a ‘volume of fluid’ type method for the interface capturing and a force-balanced continuum surface force model for the surface tension on adaptive unstructured meshes. The numerical framework is validated against experimental measurements of a droplet impact problem and is then used to study the droplet deposition onto a flowing liquid film at atmospheric and high pressure conditions. Detailed complex interfacial structures during droplet impact are captured during the simulation, which agree with the experimental observations, demonstrating the capability of the present method. It is found that the effect of the ambient pressure on the fluid properties and interfacial tension plays an important role in the droplet deposition process and the associated interfacial phenomena.


Transient freezing of molten salts in pipe-flow systems: Application to the direct reactor auxiliary cooling system (DRACS)

January 2017

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586 Reads

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24 Citations

Applied Energy

The possibility of molten-salt freezing in pipe-flow systems is a key concern for the solar-energy industry and a safety issue in the new generation of molten-salt reactors, worthy of careful consideration. This paper tackles the problem of coolant solidification in complex pipe networks by developing a transient thermohydraulic model and applying it to the ‘Direct Reactor Auxiliary Cooling System’ (DRACS), the passive-safety system proposed for the Generation-IV molten-salt reactors. The results indicate that DRACS, as currently envisioned, is prone to failure due to freezing in the air/molten-salt heat exchanger, which can occur after approximately 20 minutes, leading to reactor temperatures above 900 °C within 4 hours. The occurrence of this scenario is related to an unstable behaviour mode of DRACS in which newly formed solid-salt deposit on the pipe walls acts to decrease the flow-rate in the secondary loop, facilitating additional solid-salt deposition. Conservative criteria are suggested to facilitate preliminary assessments of early-stage DRACS designs. The present study is, to the knowledge of the authors, the first of its kind in serving to illustrate possible safety concerns in molten-salt reactors, which are otherwise considered very safe in the literature. Furthermore, and from a broader prospective, the analytical tools developed in this study can also be applied to examine the freezing propensity of molten-salt flows in other complex piping systems where standard, finite element approaches are computationally too expensive.


On the role of buoyancy-driven instabilities in horizontal liquid–liquid flow

October 2016

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115 Reads

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21 Citations

International Journal of Multiphase Flow

Horizontal flows of two initially stratified immiscible liquids with matched refractive indices, namely an aliphatic hydrocarbon oil (Exxsol D80) and an aqueous-glycerol solution, are investigated by combining two laser-based optical-diagnostic measurement techniques. Specifically, high-speed Planar Laser-Induced Fluorescence (PLIF) is used to provide spatiotemporally resolved phase information, while high-speed Particle Image and Tracking Velocimetry (PIV/PVT) are used to provide information on the velocity field in both phases. The two techniques are applied simultaneously in a vertical plane through the centreline of the investigated pipe flow, illuminated by a single laser-sheet in a time-resolved manner (at a frequency of 1 – 2 kHz depending on the flow condition). Optical distortions due to the curvature of the (transparent) circular tube test-section are corrected with the use of a graticule (target). The test section where the optical-diagnostic methods are applied is located 244 pipe-diameters downstream of the inlet section, in order to ensure a significant development length. The experimental campaign is explicitly designed to study the long-length development of immiscible liquid–liquid flows by introducing the heavier (aqueous) phase at the top of the channel and above the lighter (oil) phase that is introduced at the bottom, which corresponds to an unstably-stratified “inverted” inlet orientation in the opposite orientation to that in which the phases would naturally separate. The main focus is to evaluate the role of the subsequent interfacial instabilities on the resulting long-length flow patterns and characteristics, also by direct comparison to an existing liquid–liquid flow dataset generated in previous work, downstream of a “normal” inlet orientation in which the oil phase was introduced over the aqueous phase in a conventional stably-stratified inlet orientation. To the best knowledge of the authors this is the first time that detailed spatiotemporally resolved phase and velocity data have been generated by advanced measurement techniques in such experiments, specifically devoted to the study of long-length liquid–liquid flow development. In particular, the change in the inlet orientation imposes a Rayleigh–Taylor instability at the inlet. The effects of this instability are shown to persist along the tube, increasing the propensity for oil droplets to appear below the interface. Generally, the characteristics of the flows generated with the two inlet orientations are found to be comparable, although only six flow regimes are identified here, as opposed to eight for the original “normal” inlet orientation. The unobserved regimes are: (1) three-layer flow, and (2) aqueous-solution dispersion with an aqueous solution film. Furthermore, similar mean axial-velocity profiles are observed in the current study to those reported for the corresponding “normal” inlet orientation liquid–liquid flows. These findings are important to consider when interpreting published data from experiments performed in laboratory environments and attempting to draw conclusions relating to applications in the field. The generated data promote not only a qualitative, but importantly and uniquely, a quantitate understanding of the role of multiple instabilities on the development of these complex interfacial flows, with detailed insight into how the deviations manifest at distances 244 pipe-diameters downstream of the inlet (from high level information such as regime maps, to detailed flow information such as phase and velocity profiles). The data can be used directly for the development and validation of advanced multiphase flow models that require such detailed information.


Citations (70)


... Accurate acquisition of the degree of permeability helps us to comprehensively understand the detailed process of material transport in geological porous media, reduce the application uncertainty, and improve the application efficiency in order to more precisely evaluate and predict the dynamic characteristics of geological bodies (Bultreys et al., 2016). The traditional methods of permeability measurement require a pressure test on a core sample, and then the permeability is calculated according to Darcy's law (Manley, Steindl, Hewitt, & Bismarck, 2020). However, the test method is cumbersome and subject to environmental disturbance, and thus it is not suitable for obtaining the permeability of porous media encountered in large quantities. ...

Reference:

Transformer-based deep learning models for predicting permeability of porous media
An integrated method for measuring gas permeability and diffusivity of porous solids
  • Citing Article
  • April 2020

Chemical Engineering Science

... These peaks were not indicated and described by the classical description of Collier, Collier and Thome and Kandlikar in their flow boiling maps for similar conditions. Interestingly, these near zero vapor quality peaks were also observed in the experimental study by Kandlbinder (1997) and Urso et al (2002) [6], [7] for vertical tubes at similar low heat flux and pressure conditions. A possible explanation for the observed peak in heat transfer coefficient near zero vapor quality, is that, during flow boiling, as the liquid is being heated in the subcooled region, poor vapor nucleation as a result of low heat flux causes the tube walls and the fluid adjacent to it to heat up gradually. ...

Flow Boiling at Low Mass Flux
  • Citing Conference Paper
  • January 2002

... In contrast to the conventional picture described above, some of the recent data (Urso et al., 2002) obtained with a large diameter tube shows that, under certain conditions, the experimental data may present a somewhat different picture. Figure 2, for example, shows a region of decreasing heat transfer coefficient with respect to increasing vapour quality for higher heat fluxes in the low vapour quality range of up to 15% quality. ...

Heat Transfer at the Dryout and Near-Dryout Regions in Flow Boiling
  • Citing Conference Paper
  • January 2002

... This induces a difficulty to discern between the traces left by the liquid slugs and the elongated bubbles. One also notes that due to the effect of the buoyancy in horizontal and inclined pipes, the gas distribution within liquid slugs is not homogenously distributed along the radial direction [18], and the shape of the gas/liquid interface can be strongly distorted [120]. This further complicates the detection of the liquid slugs. ...

Introduction to Multiphase Flow: Basic Concepts, Applications and Modelling
  • Citing Book
  • January 2018

... Different flow rates or different materials result different flow regime which are topological configurations [8]. Figure 12 illustrates the vertical gas/liquid flow patterns. As the ratio of gas to liquid flow rate is increased the transformation is from bubbly flow to disperse flow [9][10][11][12]. [13] In this study the slug and the churn flow regimes were observed. ...

Flow Regimes
  • Citing Chapter
  • January 2018

... feed inlet devices [4]) are still accepted design 'components'. Numerical simulations of two-phase feeds are increasingly preferred over costly experimental investigations [7,8] and difficult 3D flow analysis [9,10]. Meanwhile, large computational power more and more allows resolving small-scale flow structures in large-scale industrial applications using models for interfacial transfer and appropriate closures [11]. ...

Modelling Strategies and Two-Phase Flow Models
  • Citing Chapter
  • January 2018

... The complexity of droplet impacts increases when considering moving surfaces, as seen in everyday phenomena like raindrops hitting a moving car windshield. Unlike the extensive literature available on droplet impact onto stationary surfaces, few works have investigated the dynamics of droplets impinging on moving dry [43][44][45] and wet walls 39,46,47 . For oblique impacts, the dynamics become more complex due to asymmetry and sliding motion. ...

Numerical study of three-dimensional droplet impact on a flowing liquid film in annular two-phase flow
  • Citing Article
  • April 2017

Chemical Engineering Science

... www.nature.com/scientificreports/ it is seen to be implemented in a variety of work, which includes but not limited to impinging jets [11][12][13] , flame speed augmentation 14 , and energy harvesting 15,16 . Square fractal grids (SFG) in particular have been extensively researched due to its distinctive nature of turbulence production and decay regions 13,17 . It was reported that the turbulence generated from multilength-scale fractal grids attained higher turbulence intensities and local flow Reynold number Re with reference to a typical regular grid of similar or higher blockage ratio σ r 10,18 . ...

Heat transfer in production and decay regions of grid-generated turbulence
  • Citing Article
  • June 2017

International Journal of Heat and Mass Transfer

... [7] In the NEUP project, a one-dimensional salt freezing model was developed and tested, which largely formed the basis of the work presented in this paper. In another study by Le Brun et al., [8] a onedimensional salt flow and freezing model was developed and validated with water freezing experimental data. The model was then applied to study possible failure modes of the direct reactor auxiliary cooling system (DRACS) of a molten salt reactor design. ...

Transient freezing of molten salts in pipe-flow systems: Application to the direct reactor auxiliary cooling system (DRACS)

Applied Energy

... This leads to instability-driven flow within the cavity. Instability occurs when the orientation of the inlet changes for horizontal liquid-liquid flow (Morgan et al., 2017). Singh & Singh (2019) investigated the natural convection heat transfer of a micropolar fluid flow between two vertical concentric cylinders using the Boussinesq approximation. ...

On the role of buoyancy-driven instabilities in horizontal liquid–liquid flow
  • Citing Article
  • October 2016

International Journal of Multiphase Flow