Non-dimensional Heat Release Research Articles

The phoenix series large-scale methane gas burner experiments and liquid methane pool fires experiments on water

This paper summarizes a series of large-scale outdoor and indoor LNG pool fire experiments conducted at Sandia National Laboratories in Albuquerque, New Mexico. Two outdoor LNG spills on water with resulting pool fires of 21 m and 56 m in diameter were conducted to improve hazard predictions by obtaining measurements of flame height, smoke production, and burn rate. The experimental data indicates that LNG pool fires on water display different behavior than those on land by producing less smoke. Surface emissive powers of up to 286 kW/m2, flames heights of up to 146 m, and burn rates of about 0.147 kg/m2 s were measured. Discussion is provided on the observed behavior of the two outdoor tests with regards to smoke production, wind effects, and hydrate production. The large-scale indoor experiments used a 3-m diameter gas burner with methane fuel to assess flame height to fire diameter ratios as a function of non-dimensional heat release rates for extrapolation to large-scale LNG fires. A flame height correlation was developed from this data.

Correlation of rate of gas temperature rise with mass loss rate in a ceiling vented compartment

n-heptane pool fire and industrial alcohol pool fire in a ceiling vented compartment were studied experimentally. The parameters including mass loss rate and rate of gas temperature rise were investigated. The results suggest that the rate of gas temperature rise, whose variations were highly coincident with those of the mass loss rate, minimized at the moment of extinction. The correlation of the rate of average nondimensional temperature rise with mass loss rate was established. It was found that the rate of average nondimensional temperature rise may be correlated with mass loss rate via the gas heat absorption coefficient which was found to be a quadratic function of the nondimensional heat release rate for the ceiling vented compartment under study. The present study may be of practical use for estimation of the time-dependent changes in mass loss rate from the gas temperature curves.

Pinch-off in forced and non-forced, buoyant laminar jet diffusion flames

This paper investigates the conditions under which flame pinch-off occurs in forced and non-forced, buoyant laminar jet diffusion flames. The fuel jet emerges into a stagnant air atmosphere at temperature T 0, with a velocity that varies periodically in time with non-dimensional frequency ω l and amplitude A = 0.5. We use the formulation developed by Liñán and Williams [1] based on the combination of the mass fraction and energy conservation equations to eliminate the reaction terms, that are substituted by the mixture fraction Z and the excess-enthalpy H scalar conservations equations. With this formulation, valid for arbitrary Lewis numbers, the flame lies on the stoichiometric mixture fraction level surface Z = Z s and its temperature can be easily calculated as T e ′ / T 0 = 1 + γ ( 1 + H s ) , where Z s = 1/(1 + S), γ is the non-dimensional heat release parameter, S is the air needed to burn the unit mass of fuel and H s is the value of the excess enthalpy at the flame surface. Non-modulated flames ω l = 0 subjected to a gravity field g are known to flicker at a non-dimensional frequency ω l,0 that depends on the Froude number Fr l . The surface of the flame is deformed by the buoyancy-induced oscillations and, for Froude numbers below a certain critical value Fr l , c ∞ , the flame breaks repeatedly in two different combustion regions (pinch-off). The first one remains attached to the burner and constitutes the main flame. The second region detaches from the tip of the flame, forming a pocket of hot gas surrounded by a flame that travels along the downstream coordinate z with velocity u ¯ ∼ ( γ z / Fr l 2 ) 1 / 2 until the fuel inside the pocket is depleted. Pinch-off is affected by the modulation of the velocity of the jet, changing the critical Froude number of pinch-off Fr l, c as the excitation frequency ω l is modified. Very large ω l / ω l,0 ≫ 1 or very small ω l / ω l,0 ≪ 1 excitation frequencies do not modify Fr l, c and it remains equal to Fr l , c ∞ . For ω l / ω l,0 ∼ O(1), the response of the flame is determined by the ratio l/ x g = γ/ Fr l , where l represents the flame length and x g is the distance at which buoyancy effects become important. A strong resonance is observed at ω l ∼ ω l,0 if the flame is sufficiently long, giving Fr l, c that could be thirty times larger than Fr l , c ∞ . Short flames do not present that peak and Fr l, c remains almost independent of ω l .

Flame Height of a Turbulent Diffusion Jet Flame with a Comparatively High Initial Discharging Velocity

A series of experiments was carried out in order to investigate the visible flame height of a jet flame with a high initial discharging velocity where the fuel-release time and mass flow rate of fuel were varied. The values of non-dimensional heat release rate (Q*) in present experiment were in the range of 107 Q* 8. Even if the value of Q* was larger than 106, when the non-dimensional number (RM) employed by Heskestad which implies the ratio of gas release momentum to the momentum made by a purely buoyant diffusion flame was less than 0.1, the flame height with a comparatively high initial discharging velocity increased with the heat release rate. In other words, whether flame height depends on the Q*2/5 or not can be decided by the range of momentum ratio (RM) regardless of the range of non-dimensional heat release rate (Q*). This can be also decided by the fire Froude number employed by Delichatsios.

An experimental study on the effect of ventilation velocity on burning rate in tunnel fires—heptane pool fire case

The 1/20 reduced-scale experiments using Froude scaling are conducted to investigate the effect of longitudinal ventilation velocity on the burning rate in tunnel fires. The n-heptane pool fires with heat release rate ranging from 3.71 to 15.6 kW are used in this study. A load cell is used to measure the mass loss rate of burning fuel and the temperature distributions are measured by K-type thermocouples in order to investigate smoke movement. The ventilation velocity in the reduced-scale tunnel is controlled by the wind tunnel through an inverter. The increases in ventilation velocity lead to enhance burning rate of n-heptane fuel. The reason is that the oxygen supply effect prevails rather than the cooling effect as the ventilation velocity increases. As a result, the heat release rates in experiment are larger than constant heat release rates by 4.45–11.3 times in the n-heptane pool fires. Also, it is found that non-dimensional critical ventilation velocity is proportional to one-third power of non-dimensional heat release rate.

Modeling Acceleration Effects on Ram Accelerator Thrust at High Pressure

A = tube section a = acceleration cp = isobaric heat capacity F = thrust h = specific enthalpy I = nondimensional thrust, F/(p A) L = length M = Mach number m = mass P = pressure ratio p = pressure Q = nondimensional heat release, q/cpT R = gas constant T = temperature u = velocity v = specific volume α = intermediate calculation parameter, LCVap = adiabatic specific heat capacity ratio γ = specific heat capacity ratio q = heat release η = calorific imperfection, h/cpT σ = compressibility factor

An Evaluation of the Global Equivalence Ratio Concept for Compartment Fires: Data Analysis Methods

An experimental effort at Virginia Tech was initiated with the aim to evaluate the applicability of the Global Equivalence Ratio (GER) concept to compartment fires with prototypical building features. The present paper contains an in depth review of previous studies, which are contrasted with a new data set. The data are presented as integrate average mole fractions exiting the compartment correlated versus a new nondimensional heat release rate. In addition to presenting good correlations for depleted oxygen, carbon dioxide, carbon monoxide, and unburned hydrocarbons, the new parameter provides an indication of the degree of burning occurring up to the compartment exit plane.

One-dimensional overdriven detonations with branched-chain kinetics

The dynamics of time-dependent, planar propagation of gaseous detonations is addressed on the basis of a three-step chemistry model that describes branched-chain processes. Relevant nondimensional parameters are the ratio of the heat release to the thermal enthalpy at the Neumann state, the nondimensional activation energies for the initiation and branching steps, the ratio of the branching time to the initiation time and the ratio of the branching time to the recombination time. The limit of strong overdrive is considered, in which pressure remains constant downstream from the leading shock in the first approximation, and the ratio of specific heats γ is taken to be near unity. A two-term expansion in the strong overdrive factor is introduced, and an integral equation is derived describing the nonlinear dynamics and exhibiting a bifurcation parameter, the reciprocal of the product of (γ−1), the nondimensional heat release and the nondimensional branching activation energy, with an acoustic correction. A stability analysis shows that, depending on values of the parameters, either the mode of lowest frequency or a mode of higher frequency may be most unstable. Numerical integrations exhibit different conditions under which oscillations die, low-frequency oscillations prevail, high-frequency oscillations prevail, highly nonlinear oscillations persist, or detonation failure occurs. This type of parametric analysis is feasible because of the relative simplicity of the model, which still is more realistic than a one-step, Arrhenius chemical approximation. In particular, by addressing the limit of slow radical recombination compared with branching, explicit results are derived for the critical value of the bifurcation parameter, involving the ratio of the recombination time to the induction time. The results help to clarify the general nature of one-dimensional detonation instability and provide simplifications that can be employed in efficiently relating gaseous detonation behavior to the true underlying chemistry.

A volumetrically heated jet: large-eddy structure and entrainment characteristics

We report here an experimental study of a round vertical liquid jet that, after achieving a self-preserving state, is subjected to volumetric heating between two diametral stations. The heat injection is achieved by applying a voltage across the stations, the jet fluid having been rendered electrically conducting by the addition of acid. Using laser-induced fluorescence, digital image processing and laser-Doppler anemometry, the flow properties of the jet have been studied in detail. It is found that, with sufficient heating, the jet no longer grows linearly with height, and the decay of both centreline velocity and turbulence intensity is arrested, and may even be reversed just beyond the zone of heat addition; nevertheless the entrainment decreases, which is at variance with the hypotheses often made for modelling it. This behaviour is here attributed to the disruptive influence that, as the present experiments show, the volumetric heating has on the large-scale vortical structures in the jet, which are known to be largely responsible for the engulfment of ambient fluid that is the first step in the entrainment process. It is shown that a new non-dimensional heat release number correlates the observed data on changes in jet width. An integral model that would describe the effect of local heating is proposed, and implications for cloud development in the atmosphere are discussed.