Hecong Liu

City University of Hong Kong | CityU

Micron-sized iron particles are promising carbon-free fuels, which can provide stable and clear energy by combustion. Micro-explosion, a phenomenon that the burning particle breaks into several fragments, can change the burn rates and size distribution of combustion products. However, the underlying mechanisms are still unclear. In this work, iron...

Flow visualization has long played a crucial role in understanding complex fluid dynamics, especially reactive flows, but traditional techniques face limitations in realizing the measurement of multiple dimensionality and multiple physical parameters. In this perspective, we introduce computational flow visualization, a new paradigm that combines o...

Linear tomographic absorption spectroscopy (LTAS) is a non-destructive diagnostic technique widely employed for gas sensing. The inverse problem of LTAS represents a classic example of an ill-posed problem. Linear iterative algorithms are commonly employed to address such problems, yielding generally poor reconstruction results due to the incapabil...

Micron-sized iron particles are promising carbon-free fuels. Efforts have been made to understand the combustion process of pure iron particles. However, it is not well-established how important are the impurities and their influence on combustion. The ignition and combustion characteristics of iron particles with 6.18 wt.% silicon were studied bot...

Laser absorption tomography (LAT) is an imaging technique used to monitor gas flow and combustion processes. The measurement plane is typically discretized using square pixels in which the flow field is assumed to be uniform. This approach can introduce large discretization errors compared to a finite element mesh with linear interpolation, especia...

Metal fuel is one of the attractive alternative fuels for its high energy density and zero carbon emission. In the past, they were often used as additives in fireworks and propellants. More attentions have been paid to metal fuels as the environmental issue and energy dilemma become increasingly severe. Ongoing efforts have been devoted to both mod...

This is a comprehensive, critical, and pedagogical review of volumetric emission tomography for combustion processes. Many flames that are of interest to scientists and engineers are turbulent and thus inherently three-dimensional, especially in practical combustors, which often contain multiple interacting flames. Fortunately, combustion leads to...

Azimuthally forced spinning modes on a single ethylene-air swirl flame in an annular combustor were investigated using phase-averaged three-dimensional (3D) Computed Tomography of Chemiluminescence. Strongly spinning azimuthal modes in the clockwise (CW) and anticlockwise (ACW) directions were introduced by means of carefully controlled acoustic fo...

Due to the unique advantages of high specific impulse, high thrust, simplicity and reliability, solid propellant with metal powder additives has emerged as a promising energy carrier for rockets in aerospace engineering. In-situ characterization of the metal particle combustion is crucial for the validation of numerical models and optimization of t...

Computed tomography imaging spectrometry (CTIS) is a snapshot spectral imaging technique that relies on a limited number of projections of the target data cube (2D spatial and 1D spectral), which can be reconstructed via a delicate tomographic reconstruction algorithm. However, the restricted angle difference between the projections and the space d...

Volumetric tomography is a powerful tool that employs 2D projections to reconstruct unknown physical quantities in 3D fluid flows. A mathematical model of the imaging process is required to recover the desired volumetric fields. Errors in the imaging model can corrupt the reconstructions, so an accurate model is critical. This work reports the firs...

An experimental investigation of the dynamic response of diffusion flames under the high voltage direct current electric field was conducted. The flame height, volume, and surface area are extracted from the three-dimensional reconstructions. The results indicate that the dynamic responses for the liquid fuel flame and the gas fuel flame show entir...

Flame emission tomography is a promising tool for combustion diagnostics. Up to now, this technique relies on the assumption that the distribution within a voxel is uniform. Such an assumption suffers from a large gradient between adjacent voxels and the loss of information within the voxel. Hence, this work aims to develop a method based on the fi...

Tomographic absorption spectroscopy (TAS) has an advantage over other optical imaging methods for practical combustor diagnostics: optical access is needed in a single plane only, and the access can be limited. However, practical TAS often suffers from limited projection data. In these cases, priors such as smoothness and sparseness can be incorpor...

The control of combustion is a hot and classical topic. Among the combustion technologies, electric-field assisted combustion is an advanced technology that enjoys major advantages such as fast response and low power consumption compared with thermal power. However, its fundamental principle and impacts on the flames are complicated due to the coup...

Recent research has shown that the electric field could impact the flame in multiple aspects, especially in flame propagation and diffusion. However, the available literature all focused on one- or two-dimensional flow characteristics and the flame under study was simple, such as a Bunsen flame or a flat flame. In this work, a method aiming to meas...

Diagnostics tools are the underpinnings for the experimental study of combustion phenomena. The inherent dynamic and three-dimensional (3-D) nature of turbulent flames has imposed strict requirements to the measurement techniques, which should provide both temporally and spatially resolved information of the target flames. Time-resolved volumetric...

Nonlinear tomography can be combined with line-of-sight measurement techniques and take the full advantage of the nonlinear relationship between the target fields to be reconstructed and the measured projections. For example, it has been integrated with absorption spectroscopy for simultaneous imaging of temperature and species concentration, and t...

Volumetric imaging represents one of the major development trends of flow diagnostics due to both the advancement in hardware and the requirement for more information to further understand complicated turbulent and/or reactive flows. Background-oriented Schlieren tomography (BOST) has become increasingly popular due to its experimental simplicity....

Multidimensional imaging has become one of the major developing trends of combustion diagnostics, and volumetric tomography is one of those techniques that has experienced significant progress over the past decades. Numerous time-resolved modalities of volumetric tomography have been developed to image a variety of physical quantities. Due to formi...

With the growing applications of nanofluid flame, the monitoring and controlling of its combustion process is of paramount importance. Thus, it is necessary to develop diagnosing methods which can simultaneously image important parameters such as temperature and volume fractions of soot, metal-oxide nanoparticles. Tomographic emission spectroscopy...

Due to the asymmetric and dynamic nature of turbulent flames encountered in practical combustors, three-dimensional measurements have been long desired to resolve the complex flame structures. Volumetric tomography has been demonstrated to be a powerful tool for this purpose. However, for practical applications, typically only a few high-speed came...

Optical tomography has become an indispensable tool for combustion diagnostics due to its noninvasiveness. However, for a typical tomography system, multiple high-speed cameras are usually required to capture different perspectives simultaneously, which is costly and requires precise synchronization, especially for the diagnostics of supersonic/hyp...

Due to the recent progresses in sensor and laser technologies, volumetric tomography has become a popular diagnostic for combustion studies. Background-oriented Schlieren tomography (BOST) is one promising modality of volumetric tomography due to its ease of implementation and versatility in imaging multiple scalars. It has been demonstrated for bo...

Computed tomography of chemiluminescence (CTC) is one kind of volumetric tomography which can recover 3D flame structures and has found extensive applications for spatiotemporally resolved measurements of flames. However, the existing CTC techniques rely on the pinhole model and fail when the flames are confined within a cylindrical glass due to im...

Online in situ prediction of 3-D flame evolution has been long desired and is considered to be the Holy Grail for the combustion community. Recent advances in computational power have facilitated the development of computational fluid dynamics (CFD), which can be used to predict flame behaviours. However, the most advanced CFD techniques are still...

China has huge potential on biogas resources and the distributed energy system (DES) based on micro gas turbine provides an effective way for biogas utilization. In this work, turbulent combustion characteristics in the combustor of micro gas turbine for DES were numerically and experimentally studied, and the effect of carbon dioxide content in bi...

Due to the development of the sensors and computer technology, combustion diagnostics based on 3D flame imaging have become indispensable tools which can reveal more information compared with planner flame imaging. Background-oriented Schlieren (BOS) tomography is one of 3D diagnostics, whose experimental setup is simpler as no expensive excitation...

Volumetric tomography is an attractive optical imaging technique for the measurement of turbulent reactive flow and has been developing rapidly in recent years. Compared with traditional planar tomography, this technique is capable of recovering evolving flow field with high fidelity so as to provide more information for relevant researches. Howeve...

The development of laser and sensor technologies have provided unprecedented opportunities for the extended applications of volumetric tomography. The recent progresses in computed tomography of chemiluminescence (CTC) have facilitated the understanding of turbulent flows and combustion instability. However, the current demonstrations of CTC can on...

Tomographic velocimetry as a 3D technique has attracted substantial research interests in recent years due to the pressing need for investigations of complex turbulent flows, which are inherently inhomogeneous. However, tomographic velocimetry usually suffers from high experimental costs, especially due to the formidable expenses of multiple high-s...

Volumetric tomography has become an indispensable tool for flow diagnostics. However, it usually suffers from high experimental costs as multiple cameras are required in a typical tomographic system. Plenoptic imaging (PI) is a promising alternative which can simultaneously record spatial and angular information using only one single camera. Althou...

Time-resolved three-dimensional diagnostics are experiencing rapid development recently due to the pressing need for the understanding of highly turbulent flows/flames that are usually encountered in next-generation gas-turbine engines or ramjets/scramjets. However, the volumetric measurements typically requires multiple high-speed cameras which ar...

Computed tomography of a chemiluminescence (CTC) system was implemented to provide time-resolved 3D measurements of an unconfined turbulent swirl flame. This system was designed in a cost-effective manner and employed three customized view registration assemblies to simultaneously capture eight projections of the target flame at a repetition rate o...

Nonlinear tomographic absorption spectroscopy (NTAS) is an emerging gas sensing technique for reactive flows that has been proven to be capable of simultaneously imaging temperature and concentration of absorbing gas. However, the nonlinear tomographic problems are typically solved with an optimization algorithm such as simulated annealing which su...

Volumetric tomography for combustion diagnostics is experiencing significant progress during the past few years due to its capability of imaging evolving turbulent flows. Such capability facilitates the understanding of the mechanisms behind complicated combustion phenomena such as lean blowout, acoustic oscillations, and formation of soot particle...

Three-dimensional computed tomography of chemiluminescence (CTC) for combustion diagnostics is attracting a surged research interest due to recent progress in sensor technologies and reduced costs of high-speed cameras. For example, it has been applied to recover the 3D distributions of intermediate chemical species such as CH* and OH*, heat releas...

Three-dimensional imaging techniques have experienced a surge in research interest during the past few years due to advancements in both hardware, i.e., the sensor arrays and data acquisition systems, and new imaging concepts, such as light field imaging and compressed sensing. Computed tomography of chemiluminescence (CTC) is an intriguing techniq...