Karl V. Meredith

A Numerical Model for Simulation of Thin-Film Water Transport over Solid Fuel Surfaces

A model for simulating water film transport over solid fuel surfaces has been developed. The fundamental film-transport equations for mass continuity, momentum, and energy were formulated. These equations have been implemented in OpenFOAM along with essential source terms for inter-phase transport. The model has been coupled to a gas-phase solver, solid boundary condition, and spray transport model. Initial validation of the model has been performed and good agreement is seen with the Nusselt solution for continuous film flows over inclined surfaces. Comparison of the film model was also made with experimental measurements for film thickness, velocity, and mass flow rate.

An Experimental Study of Fire Suppression Physics for Sprinkler Protection

An experimental study was conducted to investigate the key physics of sprinkler-based fire suppression and associated water-film transport. The objective was to evaluate experimental methods for their appropriateness in studying the fundamental physics, and provide validation data for numerical modeling. The numerical model is currently under development to simulate sprinkler-based suppression of largescale, rack-storage fires. Individual experimental techniques were explored to study water absorption, surface flow, evaporation, and suppression on vertically arranged, corrugated cardboard surfaces. In addition, water transport was investigated in full-scale rack storage configurations. The experimental results show that the tested experimental techniques are appropriate to study the key phenomena related to sprinkler-based fire suppression.

A numerical model for partially-wetted flow of thin liquid films

A model for simulating thin liquid film transport over solid surfaces has been developed.The film transportfor mass continuityand momentumwere formulated as a two-dimensional set of equations using thin-film assumptions. These equations have been implemented in an open-source CFD code (OpenFOAM). Treatments for partial-wetting phenomena have been included in the model to account for the behavior near the contact-line. A surface-tangential force along the contact line has been developed to allow for the simulation of rivulets and dry patches in two-dimensional surface flow. An approach for applying contact angle effects to the model for a real stochastic surface is outlined. Additionally, experimental measurements were made for film flow over an inclined surface for a wide range of flow rates. Using these experimental results, the model has been validated for partially wetted flow over an inclined panel. The critical flow rate of a film over a given surface was used as validation for the model. Results show that for flow rates below the critical flow rate, the partially wetted behavior of the flow was reproduced. Comparisons to experimental flow patterns and wetted-area fractions were made.

Evaporation and heat transfer from thin water films on vertical panels in simulated fire conditions

Understanding and quantifying the interaction of water films and external radiation on vertical walls is critical for fire suppression model validation. In this study, evaporation rates and surface temperatures on heated stainless steel (SS) and corrugated paperboard (CP) vertical panels were measured. External radiant heat flux was provided by an intermediate scale calorimeter (ICAL). Heat flux levels ranged between 7 and 46 kW/m 2 and were controlled by varying the distance between the ICAL and the target panel. Water flows ranging between 70 and 1900 ml/min were used in this study. Surface temperatures were measured by a custom calibrated long-wave infrared (IR) camera. Thermocapillary instabilities caused water films to quickly break down into individual rivulets when exposed to external radiation. The maximum surface temperatures on the SS panels were lower than those measured on CP panels due to more effective lateral conduction. The results also showed that water flows on SS surfaces are more effective in extracting heat than similar water flows on CP panels. The experimental results are expected to help understand multi-phase heat transfer for surface flows in fire environments, and to provide validation data for developing numerical models.

Rupture of thin liquid films under the influence of external heat flux

A thin liquid film completely covering a solid surface has the potential to rupture into rivulets when exposed to external heat flux due to thermocapillary instabilities and vaporization. This has implications for many industrial applications, but particularly for fire suppression. Film rupture drastically reduces the wetted surface area thereby exposing the solid surface to large incident heat fluxes. Recently, a computational fluid dynamics (CFD) model‐FireFOAM‐has been developed for simulating fire suppression phenomena. The purpose of this study is to investigate and validate FireFOAM’s thin film model for predictions of critical heat flux required to induce film rupture. To that extent, vertically flowing film experiments with flow rates ranging form 8.7 g/m/s to 41.3 g/m/s and external heat flux extending from 5 kW/m 2 to 45 kW/m 2 have been simulated. The film heats up non-uniformly and eventually ruptures due to thermocapillary instabilities. Dry regions are formed on the panel as the film pulls together as rivulets. For a given flow rate, the radiative heat flux was varied to identify the ‘critical heat flux’. Good qualitative and quantitative match was achieved between model and experiments. At the lower flow rates, even a slight incident heat flux would cause the film to break into rivulets. As the flow rate increased, stronger and stronger heat flux values were necessary to cause film rupture. At very high flow rates, the film remained continuous over the range of heat flux tested. Film rupture was shown to be sensitive to the film inlet conditions.