Yang-Cheng Shih

Dynamic airflow simulation within an isolation room

Abstract In many hospitals, isolation rooms are used to contain patients who are highly infectious, and the spread of air and bacteria within the isolation room is closely relates to room air distribution. This article uses the computational fluid dynamics (CFD) method to investigate the effects of a moving person and the opening and closing of a sliding door on room air distribution, including velocity, pressure and contaminant fields. Dynamic meshes are employed to simulate the movement of the walking person and sliding door. According to numerical results, the impact of those moving objects on room air distribution is addressed in this study.

Periodic Fluid Flow and Heat Transfer in a Square Cavity Due to an Insulated or Isothermal Rotating Cylinder

The periodic state of laminar flow and heat transfer due to an insulated or isothermal rotating cylinder object in a square cavity is investigated computationally. A finite-volume-based computational methodology utilizing primitive variables is used. Various rotating objects (circle, square, and equilateral triangle) with different sizes are placed in the middle of a square cavity. A combination of a fixed computational grid and a sliding mesh was utilized for the square and triangle shapes. For the insulated and isothermal objects, the cavity is maintained as differentially heated and isothermal enclosures, respectively. Natural convection heat transfer is neglected. For a given shape of the object and a constant angular velocity, a range of rotating Reynolds numbers are covered for a Pr = 5 fluid. The Reynolds numbers were selected so that the flow fields are not generally affected by the Taylor instabilities (Ta <1750). The periodic flow field, the interaction of the rotating objects with the recirculating vortices at the four corners, and the periodic channeling effect of the traversing vertices are clearly elucidated. The simulations of the dynamic flow fields were confirmed against experimental data obtained by particle image velocimetry. The corresponding thermal fields in relation to the evolving flow patterns and the skewness of the temperature contours in comparison to the conduction-only case were discussed. The skewness is observed to become more marked as the Reynolds number is lowered. Transient variations of the average Nusselt numbers of the respective systems show that for high Re numbers, a quasiperiodic behavior due to the onset of the Taylor instabilities is dominant, whereas for low Re numbers, periodicity of the system is clearly observed. Time-integrated average Nusselt numbers of the insulated and isothermal object systems were correlated with the rotational Reynolds number and shape of the object. For high Re numbers, the performance of the system is independent of the shape of the object. On the other hand, with lowering of the hydraulic diameter (i.e., bigger objects), the triangle and the circle exhibit the highest and lowest heat transfers, respectively. High intensity of the periodic channeling and not its frequency is identified as the cause of the observed enhancement.

Theoretical Modeling of Intumescent Fire-Retardant Materials

A theoretical model is developed to predict the thermochemical be havior of intumescent fire-retardant coatings. The model is based on the assump tion that the intumescent reaction is analogous to the phase change process occurring over a finite temperature range. From the numerical results, it is found that the histories of the substrate temperature can be accurately predicted by choosing adequate pseudo latent heat and temperature range for the intumes cent reaction, and the bending evidence observed in experiments can be success fully predicted by the present intumescence model. Finally, it is shown that the present model can readily be extended to simulate the intumescent process with multi-intumescent zones.

Numerical study on the dispersion of airborne contaminants from an isolation room in the case of door opening

Abstract A negative pressure isolation room is built to accommodate and cure patients with highly infectious diseases. An absolutely airtight space effectively prevents infectious diseases from leaking out of the isolation room. Opening the door leads to a breakdown in isolation conditions and causes the dispersion of infectious air out of the isolation room. Extensively employed to manage smoke in cases of fires at subway and highway tunnels, a concept of controlling airflow is applied to the study. This study proposes a design of ventilation system to control air flow rate for containing airborne contaminant and preventing its spread to the adjacent rooms when the door to the isolation room is opened and closed. This paper employs computational fluid dynamics (CFD) as a more effective approach to examine the concentration maps of airborne contaminants and the airflow patterns of room air and discuss the influence of temperature differences between two rooms on airborne dispersion. Results show that an air velocity above 0.2 m/s via a doorway effectively prevents the spread of airborne contaminants out of the isolation room in the state of door opening.

Numerical Study of Transient Thermal Ablation of High-Temperature Insulation Materials

A physical model has been developed to describe the transient ablation phenomena of high-temperature insulation materials for the cases with and without the formation of a melt layer on the material surface. The model takes account of the effects of transient melt-layer formation, variable ablation temperatures, and heat of ablation of the material. Validity of the model has been demonstrated numerically by comparison with available analytical solutions for the special case of a constant ablation temperature. For the general case of variable ablation temperatures, appreciable differences in the predicted ablation rates have been found between the cases with and without melt-layer formation for materials having low heats of ablation and for large imposed external heat fluxes. The present study clearly indicates that the melt-layer effect cannot be neglected at high external heat fluxes, especially for materials such as MXBE-350 that have low heats of ablation.

Numerical Study of Solidification around Staggered Cylinders in a Fixed Space

ABSTRACT Solidification phase-change problems occur in a variety of industrial applications, such as systems for thermal energy storage. In this article, an enthalpy formulation-based fixed-grid approach is employed to study the solidification around a number of staggered cylinders in a fixed volume. Pure water is adopted as the working fluid and the non-Boussinesq model is used to calculate the variation in water density to account for the natural convection during solidification. The main interest of this study focuses on the effects of cylinder number and water superheat on the growth of the ice layer and the heat transfer characteristics during solidification.

NUMERICAL STUDY OF HEAT TRANSFER PERFORMANCE ON THE AIR SIDE OF EVAPORATOR FOR A DOMESTIC REFRIGERATOR

The Computational Fluid Dynamics (CFD) method has been applied to predict the heat transfer performance of evaporator on the air side for a domestic refrigerator in this article. From the numerical results, it can be shown that nonuniformities exist on the distributions of air velocity, air temperature, and local heat transfer rate. The flow maldistributions are mainly caused by the ill-designed supplier of frozen storage room. They could deteriorate the overall heat transfer performance of evaporator and may result in the refrigerant degradation on the refrigerant side.

Transient Leading to Periodic Fluid Flow and Heat Transfer in a Differentially-Heated Cavity Due to an Insulated Rotating Object

Computational analysis of transient phenomenon followed by the periodic state of laminar flow and heat transfer due to an insulated rotating object in a square cavity is investigated. A finite-volume-based computational methodology utilizing primitive variables is used. Various rotating objects (circle, square and equilateral triangle) with different sizes are placed in the middle of the cavity. A combination of a fixed computational grid with a sliding mesh was utilized for the square and triangle shapes. The cavity is maintained as a differentially-heated enclosure and the motionless insulated object is set in rotation at time t = 0. Natural convection heat transfer is neglected. For a given shape of the object and a constant angular velocity, a range of rotating Reynolds numbers are covered for a Pr = 5 fluid. The Reynolds numbers were selected so that the flow fields are not generally affected by the Taylor instabilities (Ta < 1750). The evolving flow field and the interaction of the rotating objects with the recirculating vortices at the four corners are elucidated. The corresponding thermal fields in relation to the evolving flow patterns and the skewness of the temperature contours in comparison to conduction-only case were discussed. The skewness is observed to become more marked as the Reynolds number is lowered. At the same time, similarity of the thermal fields for various shapes for the same Reynolds number varifies the appropriate selection of the hydraulic diameter. Transient variations of the average Nusselt numbers on the two walls show that for high Re numbers, a quasi-periodic behavior due to the onset of the Taylor instabilities is dominant, whereas for low Re numbers, periodicity of the system is clearly observed. Time-integrated average Nusselt number of the cavity is correlated to the rotational Reynolds number and shape of the object. The triangle object clearly gives rise to high heat transfer followed by the square and circle objects.Copyright

Numerical study of the thermal response of high-temperature ablative materials

Abstract A theoretical model accounting for the effects of thermal nonequilibrium, temperature-dependent material properties, pyrotysis reaction, and thermochemical expansion is developed to predict the thermal response of high-temperature ablative materials when exposed to hyperthermal environments. The model, which is developed by using the volume-averaging and the finite volume techniques, is applied to predict the thermal response and the underlying heat transfer mechanisms of two typical ablative materials having distinctly different properties. From the numerical results, it is found that the method of mixture enthalpy leads to a better prediction of the thermal response than the method of mixture specific heat. It is also found that for an ablative material with relatively large permeability and porosity, the cooling effect of transpiration gases is significantly overpredicted by using the assumption of local thermal equilibrium.

The Effect of Viscous Dissipation on Heat Transfer in Cavities of Varying Shape Due to an Inner Rotating Circular Cylinder

In this study, the finite-volume-based computational methodology was used to investigate the effect of a highly viscous fluid on the evolution of flow and thermal fields in cavities of different shapes, including a circle, square, and equilateral triangle, due to an inner isothermal rotating cylinder. The temperature of the cavity wall was kept constant, but differed from that of the inner cylinder. Numerical results revealed that the triangular cavity had the greatest ability to dissipate internal thermal energy through the side walls, while the circular cavity had the worst performance.