7 results found
Gupta A, Markides CN, 2020, Autoignition of an n-heptane jet in a confined turbulent hot coflow of air, Experimental Thermal and Fluid Science, Vol: 119, Pages: 1-23, ISSN: 0894-1777
The autoignition of a continuous, single jet of pure liquid n-heptane injected concentrically and axisymmetrically from a water-cooled circular nozzle into a confined turbulent hot coflow (CTHC) of air at atmospheric pressure has been investigated experimentally at air temperatures up to 1150 K and velocities up to 40 m/s. The aim of this work was to examine the emergence of liquid-fuel autoignition in the presence of flow, mixture and phase inhomogeneities, to which end, the velocity, temperature and fuel-droplet fields inside the CTHC reactor were characterized in a series of dedicated measurement campaigns. Distinct phenomena were identified concerning the emergence of various regimes: no autoignition, random spots, and continuous flame. In the random spots regime, autoignition appeared in the form of well-defined, discrete localized spots occurring randomly within the reactor, similar to observations in a similar apparatus with gaseous fuels (Markides, 2005; Markides and Mastorakos, 2005, 2011; Markides et al., 2007). High-speed optical measurements of these random spots were made from which the autoignition locations/lengths were measured, and then used to infer average autoignition delay, or residence, times from injection based on the bulk air velocity. An increase in the air temperature moved the region of autoigniting spots closer to the injector nozzle, thus decreasing the autoignition length and also decreasing the autoignition delay time. Generally, autoignition moved downstream with increasing bulk air velocity, but the delay times decreased contrary to the aforementioned earlier work with pre-vaporized n-heptane in this geometry. Of interest is the finding that at the highest investigated air velocities, the autoignition length decreased as the air velocity increased, which again deviates from the same earlier work with vaporized n-heptane. Furthermore, higher liquid injection velocities also resulted in increased autoignition lengths and times. The re
Gupta A, Qadri UA, Koutita K, et al., 2020, Experimental investigation of the flow in a micro-channelled combustor and its relation to flame behaviour, Experimental Thermal and Fluid Science, Vol: 116, ISSN: 0894-1777
The dynamic behaviour of periodic laminar premixed acetylene-air flames in a micro-channelled combustor consisting of an array of five planar rectangular channels was found to be influenced by the equiv- alence ratio and flow-rate of the continuously and steadily injected premixed fuel charge. Three distinct flame stages were observed — planar, chaotic and trident, which were strongly correlated to the flow dynamics. The effect of the flow on the flame behaviour was investigated by characterizing the cold flow in a scaled-up model channel with the same aspect ratio as the combustion micro-channel. Direct flow visualization using flow tracers and quantitative velocity-field data from PIV measurements showed both an increase in the bottom recircula- tion zone reattachment length (along the floor of the channel) and a decrease in the lateral recirculation zone reattachment length (along the sides of the channel) with increasing flow Reynolds number. Comparison of the flow and flame transition locations downstream of the injection point suggested that the location of trident flame onset coincides with the flow bottom recirculation zone reattachment length. The planar-chaotic flame transition location was observed to be influenced by the homogeneity of the mixture downstream of the injection plane.
Moran H, Gupta A, Voulgaropoulos V, et al., 2018, Autoignition of a liquid n-heptane jet injected into a confined turbulent hot co-flow, 3rd SEE SDEWES 2018, Publisher: SDEWES
Alternatives to conventional combustion engines, such as gasoline direct injection engines, homogeneous charge compression injection engines and dual-fuel turbines, promise improved fuel efficiency and reduced emissions. The present study of liquid-fuel autoignition in turbulent flows explores the underlying phenomena in these applications towards next-generation combustors. Experiments have been performed on the autoignition of continuous liquid n-heptane jets injected axisymmetrically into confined turbulent coflows of preheated air. Jet atomisation was characterised using high-speed imaging, and autoignition locations and corresponding delay times were recorded for various bulk air temperatures and velocities. Two turbulence-generating plates with different perforation sizes were used to investigate the role of turbulence in affecting the phenomena under investigation. Smaller droplets formed in flows with lower turbulence intensities and larger integral lengthscales. The autoignition length increased and delay time decreased with increasing bulk air velocity, the latter being contrary to results from pre-vaporized n-heptane autoignition in an identical apparatus.
Gupta A, Markides CN, 2017, An experimental study of the autoignition of polydispersed liquid-fuel droplets in a confined high-temperature turbulent coflow, 8th European Combustion Meeting, Publisher: Combustion Institute
We present experimental data on the autoignition of polydispersed droplets of liquid n-pentane injected axisymmet-rically from a circular nozzle into a confined turbulent coflow of hot air at atmospheric pressure, with the aim ofexamining the emergence of autoignition in the presence of flow, mixture and phase inhomogeneities. In the regime ofinterest, autoignition occurred in the form of random spots. At higher air temperatures and lower fuel injection veloc-ities, autoignition was observed closer to the injector; the corresponding delay (residence) times also decreased. Withincreasing air velocity and hence turbulent velocity fluctuations, autoignition moved downstream but the delay timesdecreased. The results are also compared to equivalent results obtained with n-heptane (from previous experiments inthe same apparatus). The data can be used for the development of advanced multiphase turbulent combustion models.
Gupta A, Mathie R, Markides CN, 2014, An experimental and computational investigation of a thermal storage system based on a phase change material: Heat transfer and performance characterization, Computational Thermal Sciences, Vol: 6, Pages: 341-359, ISSN: 1940-2554
The integration of latent heat storage solutions into modern heating and cooling systems has the potential to enhance overall system performance compared to standard hot water systems (radiators and tanks) due to an augmentation of the stored heat by the latent heat of a suitable material. This paper presents computational predictions complemented by experimental measurements of the dynamic behavior and performance of an active thermal storage system for domestic applications, based on the use of a hydrated salt phase change material (PCM) and a conventional cylindrical storage tank. The thermal storage (heating) and extraction (cooling) rates for this PCM-filled tank are compared to a water-filled tank. Flow and temperature fields are analyzed in a customized storage tank design for heat transfer and performance characterization. Experimental findings show good agreement with full 3-D simulation results. The heat removal characteristic is identified as being the main factor limiting the arrangement’s performance when compared to a water-based system, due to the solidification of the PCM onto the pipes, and a significant consequent decrease in heat flux. It is confirmed that the PCM thermal storage solution has the capability to store a large amount of heat effectively, but design improvements are required to eliminate the cooling-limited heat transfer process in the investigated arrangement.
Gupta A, Markides CN, Mathie R, 2013, Investigation of a thermal storage system based on phase change material: heat transfer and performance characterisation, 13th UK Heat Transfer Conference, UKHTC2013, Publisher: Design Engineering Group, Imperial College London, Pages: 182-1-182-8
The integration of latent heat storage solutions into modern domestic heating systems has the potentialto enhance the overall system performance compared to standard hot-water systems (radiators andtanks) due to augmentation of the stored heat by the latent heat of a suitable material. This paperpresents computational prediction and experimental validation of the dynamic behaviour andperformance of an active thermal storage system for domestic applications, based on the use of ahydrated salt PCM. The thermal extraction and heating rates for the PCM tank are compared to a waterfilled tank. Flow and temperature fields are analysed in a customised storage tank design for heattransfer and performance characterisation. Experimental findings show excellent agreement with the3D CFD simulation results. The heat removal performance has been identified as being the limitingfactor when compared to a water-based system. It is also confirmed that the PCM solution has thecapability to store a large amount of heat effectively but design improvements are required toeliminate the cooling limited heat transfer process in the current apparatus.
Markides CN, Gupta A, 2013, Experimental investigation of a thermally powered central heating circulator: Pumping characteristics, Applied Energy, Vol: 110, Pages: 132-146, ISSN: 1872-9118
A thermally powered circulator based on a two-phase thermofluidic oscillator was constructed and operated successfully as a replacement for a central heating hot water circulator coupled to a domestic gas-fired boiler. During regular operation the thermally powered circulator demonstrated a pumped flow-rate that decreased monotonically as the head applied across it increased. A maximum measured flow-rate of 850 L/h was achieved at zero head, and a maximum head of 8.4 mH2O was attained at near-stalling (zero flow-rate) conditions. In agreement with previous modelling studies of the technology, increased inertia in the load line seems to lead to improved circulator performance. Further, the oscillating circulator exhibited an operational frequency between 0.24 and 0.33 Hz, which was mostly determined by the circulator configuration. The pumping capacity was strongly affected by the oscillating liquid amplitudes in the power cylinder that defined the positive displacement amplitudes of the liquid piston into and out of the hot water circuit. The best circulator configuration was associated with lower operation frequencies and relatively large ratios of suction to discharge displacement.
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