Kun Zhao

Bio: Kun Zhao is an academic researcher from Nanjing Tech University. The author has contributed to research in topics: Flame spread & Heat flux. The author has an hindex of 7, co-authored 19 publications receiving 113 citations. Previous affiliations of Kun Zhao include University of Maryland, College Park & University of Science and Technology of China.

Downstream radiative and convective heating from methane and propane fires with cross wind

Abstract: Experiments were conducted to elucidate the radiative and convective heating occurring downstream of wind-driven fires produced by a gaseous burner. These flames model, at reduced scale, some of the dynamics observed in wind-driven fire spread through wildlands, buildings, mines or tunnels. Methane and propane were used to create fires ranging from 5 to 25 kW with ambient velocities ranging from 0.6 to 2.2 m/s. The total and incident radiative heat flux to a nearly-adiabatic downstream surface were measured by a water-cooled total heat flux gauge and a radiometer, respectively. The interaction between the buoyancy induced by the flame and momentum from the free stream was represented by a mixed-convection parameter, ξ = G r x 2 / R e x 1 n , where n = 3/2, 2 or 5/2. ξ was evaluated with two length scales in order to capture effects of both the boundary layer development length (x1) and heated distance downstream of the burner (x2). Results showed that the propane flame (high luminosity) exhibited slightly higher radiative heat fluxes than methane flames (low luminosity) under the same external conditions, while the convective heat flux followed an opposite trend. The downstream local radiative heat flux was quantified using a dimensionless flame thickness δ x * , which showed a good relationship with ξ for n = 5/2 but not 3/2 or 2. The local convective heat transfer coefficient was expressed in the form of a local Nusselt number, N u x 2 R e x 1 − 1 / 2 , and correlated well as a piecewise function with ξ for n = 5/2. It was found that both δ x * and N u x 2 R e x 1 − 1 / 2 have a turning point at ξ ≈ 0.005, which was visually shown to denote the location where transition between an attachment and plume-like flame occurs. By separately describing both radiative and convective downstream heating, the mechanisms controlling heating which drives flame spread in wind-driven fires can be further understood.

Experimental and theoretical study on downward flame spread over uninhibited PMMA slabs under different pressure environments

Abstract: This paper presents an experimental and theoretical study of side-edge effects on downward flame spread over two parallel polymethyl methacrylate (PMMA) slabs under different pressure environments. Identical experiments of downward flame spread over thin PMMA slabs with side-edges unrestrained were conducted at different altitudes in Hefei (102 kPa), Geermu (73.2 kPa) and Lhasa (66.3 kPa). Experimental results show that the flame spread rate is controlled by ignition along the side-edge, rather than at the center of the samples, for experiments with both single and two parallel slabs. Based on these results, a thermal model is developed which describes flame spread along the edge and quantitatively agrees with experimental results. In the parallel-slab case, convective heating appears to influence the spread rate only when the separation distance is very small, with radiative heating playing a more important role as separation distance increases. The angle of the pyrolysis front, formed between the faster side-edge spread and slower center-region spread, hardly changes with pressure, but changes significantly with separation distance, due to differing modes of heat transfer between the side-edge and center region. In addition, variations of flame height with pressure and separation distance are reasonably interpreted from diffusion flame theory.

Macroscopic characteristics and prediction model of horizontal extension length for syngas jet flame under inclined conditions

Abstract: As an important industrial raw material, syngas can be used for hydrogen production, and its main components include hydrogen and carbon monoxide. Experiments were conducted on syngas (ratio of CO and H2 is 1:1) to study the effects of the nozzle inclined angle and diameter as well as the heat release rate on the characteristics of syngas jet flame. Three nozzles with different diameters, 5 mm, 10 mm, and 15 mm, were used in this study. Four different inclined angles of the nozzle corresponding to 0°, 30°, 60°, and 90° were employed. It was found that the normalized horizontal extension length of jet flame (L/D) varies with nozzle diameter and fuel flow rate, and increases exponentially as the dimensionless heat release rate ( Q ∗ ) increases for the nozzle with inclined angles of 0°–60°. By modifying the dimensionless heat release rate and fitting the experimental data, a unified empirical model, which is the first of its kind to date, is established. The model can be used to estimate the horizontal extension length of the syngas jet flame for nozzles with different diameters and inclined angles. For jet flame at an inclined angle of 90°, the normalized jet flame width was exponentially proportional to the modified Froud number with a power of 0.39.

Experimental study on burning behaviors of photovoltaic panels with different coverings using a cone calorimeter

Abstract: Severe building integrated photovoltaic (BIPV) fires enhance the need of precise risk assessment on photovoltaic (PV) modules. In the current study, two widely used photovoltaic (PV) panels with different coverings are tested using a cone calorimeter under a wide range of incident heat fluxes (from 18 to 70 kW/m2) to characterize the influence of window flame radiation on the burning behaviors of the samples. Several key parameters including the ignition temperature and time, heat release rate (HRR), and concentration of toxic gases are investigated. The experimental results show that glass covering instead of polyethylene terephthalate covering could effectively improve the flame-retardant and smoke-suppression properties of PV composites. In addition, exposing the two samples to high incident heat fluxes would be very dangerous for the peak HRR and fire growth rate index, and fractional effective exposure dose values rise as the incident heat flux increases from 18 to 70 kW/m2, while the ignition time a...

Effect of orientation on the burning and flame characteristics of PMMA slabs under different pressure environments

Abstract: To investigate the combined effects of orientation and environmental pressure on flame characteristics, steady burning experiments of inclined polymethyl methacrylate (PMMA) slabs were conducted at reduced environmental pressures ranging from 0.5 to 1 atm. According to the visual observations of flame appearance and fitting results of burning rate at different inclination angles, diffusion flames can be classified into three regions: pool-fire region, transition region and wall-fire region. Experimental result shows that flame pulsation frequency depends only on sample width in pool-fire region, however, mainly on fuel inclination angle in wall-fire region. A theoretical analysis is performed to predict the flame pulsation frequency as a function of modified gravity and the outlet diameter of gas-phase flame at different fuel inclination angles. In addition, the dimensionless flame pulsation frequency St is found to correlate well with the modified Froude number Fr ∗ , St = 0.46 1 / Fr ∗ 0.53 . Flame length shows a tendency to peak at 0.8 atm except 2.5 cm wide samples. Correlations between flame length and heat release rate are found with a power-law exponent of 1.41 for pressures smaller than 0.8 atm and 1.04 for larger pressures. Both experimental result and the developed correlations of flame length suggest that the roles of molecular diffusion and air entrainment played in diffusion burning are closely related to the environmental pressure and sample dimension.

Convection Heat Transfer

Abstract: Geometry: shape, size, aspect ratio and orientation Flow Type: forced, natural, laminar, turbulent, internal, external Boundary: isothermal (Tw = constant) or isoflux (q̇w = constant) Fluid Type: viscous oil, water, gases or liquid metals Properties: all properties determined at film temperature Tf = (Tw + T∞)/2 Note: ρ and ν ∝ 1/Patm ⇒ see Q12-40 density: ρ ((kg/m) specific heat: Cp (J/kg ·K) dynamic viscosity: μ, (N · s/m) kinematic viscosity: ν ≡ μ/ρ (m/s) thermal conductivity: k, (W/m ·K) thermal diffusivity: α, ≡ k/(ρ · Cp) (m/s) Prandtl number: Pr, ≡ ν/α (−−) volumetric compressibility: β, (1/K)

A Review for Solar Panel Fire Accident Prevention in Large-Scale PV Applications

Abstract: Due to the wide applications of solar photovoltaic (PV) technology, safe operation and maintenance of the installed solar panels become more critical as there are potential menaces such as hot spot effects and DC arcs, which may cause fire accidents to the solar panels. In order to minimize the risks of fire accidents in large scale applications of solar panels, this review focuses on the latest techniques for reducing hot spot effects and DC arcs. The risk mitigation solutions mainly focus on two aspects: structure reconfiguration and faulty diagnosis algorithm. The first is to reduce the hot spot effect by adjusting the space between two PV modules in a PV array or relocate some PV modules. The second is to detect the DC arc fault before it causes fire. There are three types of arc detection techniques, including physical analysis, neural network analysis, and wavelet detection analysis. Through these detection methods, the faulty PV cells can be found in a timely manner thereby reducing the risk of PV fire. Based on the review, some precautions to prevent solar panel related fire accidents in large-scale solar PV plants that are located adjacent to residential and commercial areas.

Downstream radiative and convective heating from methane and propane fires with cross wind

Abstract: Experiments were conducted to elucidate the radiative and convective heating occurring downstream of wind-driven fires produced by a gaseous burner. These flames model, at reduced scale, some of the dynamics observed in wind-driven fire spread through wildlands, buildings, mines or tunnels. Methane and propane were used to create fires ranging from 5 to 25 kW with ambient velocities ranging from 0.6 to 2.2 m/s. The total and incident radiative heat flux to a nearly-adiabatic downstream surface were measured by a water-cooled total heat flux gauge and a radiometer, respectively. The interaction between the buoyancy induced by the flame and momentum from the free stream was represented by a mixed-convection parameter, ξ = G r x 2 / R e x 1 n , where n = 3/2, 2 or 5/2. ξ was evaluated with two length scales in order to capture effects of both the boundary layer development length (x1) and heated distance downstream of the burner (x2). Results showed that the propane flame (high luminosity) exhibited slightly higher radiative heat fluxes than methane flames (low luminosity) under the same external conditions, while the convective heat flux followed an opposite trend. The downstream local radiative heat flux was quantified using a dimensionless flame thickness δ x * , which showed a good relationship with ξ for n = 5/2 but not 3/2 or 2. The local convective heat transfer coefficient was expressed in the form of a local Nusselt number, N u x 2 R e x 1 − 1 / 2 , and correlated well as a piecewise function with ξ for n = 5/2. It was found that both δ x * and N u x 2 R e x 1 − 1 / 2 have a turning point at ξ ≈ 0.005, which was visually shown to denote the location where transition between an attachment and plume-like flame occurs. By separately describing both radiative and convective downstream heating, the mechanisms controlling heating which drives flame spread in wind-driven fires can be further understood.

Influence of vertical channel on downward flame spread over extruded polystyrene foam

Abstract: The influence of vertical channel with various structure factors (α) on downward flame spread over extruded polystyrene (XPS) foam is investigated. The flame shape, flame height (Hf), temperature in the channel (Tv) and on the curtain wall surface (Tc), flame spread rate (Vf), and induced air flow (Q) are obtained. Hf first drops and then increases as α rises, attributing to similar changing trends of induced airflow rate and Froude number. Concerning the temperature history, Tc > Tv at the initial stage while Tc T c ‾ (average value) and T v ‾ drop as α rises, and T v ‾ > T c ‾ under most conditions. A model is established to predict the total, convective and radiative heat fluxes transferred to preheating zone. The convective heat flux is more dominant than the radiative heat feedback from the curtain wall. Vf first rises and then drops as α increases. For α

Experimental and theoretical study on downward flame spread over uninhibited PMMA slabs under different pressure environments

Abstract: This paper presents an experimental and theoretical study of side-edge effects on downward flame spread over two parallel polymethyl methacrylate (PMMA) slabs under different pressure environments. Identical experiments of downward flame spread over thin PMMA slabs with side-edges unrestrained were conducted at different altitudes in Hefei (102 kPa), Geermu (73.2 kPa) and Lhasa (66.3 kPa). Experimental results show that the flame spread rate is controlled by ignition along the side-edge, rather than at the center of the samples, for experiments with both single and two parallel slabs. Based on these results, a thermal model is developed which describes flame spread along the edge and quantitatively agrees with experimental results. In the parallel-slab case, convective heating appears to influence the spread rate only when the separation distance is very small, with radiative heating playing a more important role as separation distance increases. The angle of the pyrolysis front, formed between the faster side-edge spread and slower center-region spread, hardly changes with pressure, but changes significantly with separation distance, due to differing modes of heat transfer between the side-edge and center region. In addition, variations of flame height with pressure and separation distance are reasonably interpreted from diffusion flame theory.