William P. Bahnfleth

Mathematical Modeling of Ultraviolet Germicidal Irradiation for Air Disinfection

A comprehensive treatment of the mathematical basis for modeling the disinfection process for air using ultraviolet germicidal irradiation (UVGI). A complete mathematical description of the survival curve is developed that incorporates both a two stage inactivation curve and a shoulder. A methodology for the evaluation of the three-dimensional intensity fields around UV lamps and within reflective enclosures is summarized that will enable determination of the UV dose absorbed by aerosolized microbes. The results of past UVGI studies on airborne pathogens are tabulated. The airborne rate constant for Bacillus subtilis is confirmed based on results of an independent test. A re-evaluation of data from several previous studies demonstrates the application of the shoulder and two-stage models. The methods presented here will enable accurate interpretation of experimental results involving aerosolized microorganisms exposed to UVGI and associated relative humidity effects

Demonstration of a Hermetic Airborne Ozone Disinfection System: Studies on E. coli

An enclosed flow-through system using airborne ozone for disinfection and which removes the ozone with a catalytic converter was tested with a strain of Escherichia coli. Petri dishes containing the microorganisms were inserted in a chamber and exposed for 10-480 min to ozone concentrations between 4 and 20 ppm. Death rates in excess of 99.99% were achieved. Survival data is fitted to a two-stage curve with a shoulder based on the multihit target model. Ozone was removed from the exhaust air to nondetectable levels using a metal oxide based catalyst. The possibility of using ozone as an airborne disinfectant for internal building surfaces and catalytically removing the ozone on exhaust is demonstrated to be feasible. A model for the decay of Bacillus cereus under ozone exposure is proposed as an example for predicting the sterilization of buildings contaminated with anthrax. The potential for disinfecting airstreams and removing ozone to create breathable air is also implied by the results of this experiment.

Parametric Study of Charging Inlet Diffuser Performance in Stratified Chilled Water Storage Tanks with Radial Diffusers: Part 1–Model Development and Validation

A computational fluid dynamics (CFD) model has been developed to simulate the flow and heat transfer near a lower radial inlet diffuser in a cylindrical stratified chilled water storage tank during charging. The model was used to perform parametric simulations of inlet diffuser performance in full-scale tanks during thermocline formation, from which first-order correlation relating thermal performance to tank and diffuser design parameters were derived. Part 1 describes the development of the CFD model and its validation. Part 2 reports the results of the parametric study. The two-dimensional, transient model was implemented in a commercial finite element code. The model has been validated with field-measured data from two full-scale tanks. Laminar analysis predicted field-measured temperature profiles well, even under ostensibly transitional conditions, provided that variations in inlet temperature during the field tests were accurately modeled. These results suggest that turbulence has a secondary effect on the development of thermoclines within the range of parameters considered and that laminar models may be useful for modeling performance of full-scale tanks over a wide range of inlet parameters. The fully developed temperature profile was found to be insensitive to the velocity distribution assumed at the inlet.

Parametric Study of Charging Inlet Diffuser Performance in Stratified Chilled Water Storage Tanks with Radial Diffusers: Part 2–Dimensional Analysis, Parametric Simulations and Simplified Model Development

A parametric study of the thermal performance during charging of a full-scale radial inlet diffuser in a cylindrical stratified chilled water storage tank was performed by applying factorial experimental theory to the results of simulations performed with a validated computational fluid dynamics (CFD) model. Part 1 describes the development and validation of the CFD model. Part 2 summarizes the results of the parametric study. Dimensional storage tank and inlet dif-fuser parameters having the potential to influence inlet thermal performance were identified, then formed into dimensionless groups using Buckingham Pi analysis. These included the inlet Richardson number (Ri), inlet Reynolds number (Re i ), ratio of diffuser radius to tank radius (RD /Rw ) and ratio of diffuser radius to diffuser inlet height (RD/hi). Thermal performance was measured in terms of thermocline thickness and equivalent lost tank height. Sixteen simulations comprising a full 2 k factorial experiment were completed and analyzed. Parameter ranges considered were as follows: Ri from 1.0 to 11.1, Re i from 1,000 to 12,000, RD /Rw from 0.2 to 0.4 and RD /hi from 5 to 10. Within these ranges, Ri, RD /Rw and RD /hi were found to be of first-order significance while Re i was not. Regression models of thermal performance metrics as functions of Ri, RD /Rw and RD /hi that are sufficiently simple to be useful for design were developed. These models agreed well with CFD simulations from which they were derived and with field data.

Measurements and Factorial Analysis of Micron-Sized Particle Adhesion Force to Indoor Flooring Materials by Electrostatic Detachment Method

Airborne concentration of micron-sized particulate matter (PM) is an important index of indoor air quality. While human activity is considered the main reason causing indoor particle resuspension, theoretical particle adhesion force models give predictions of adhesion force much larger than the disturbance forces introduced by human activity. This work suggests that the imperfect contact between particles and surfaces can greatly reduce the adhesion bond. Electrostatic detachment method is used to measure the actual adhesion force distribution of micron-sized particles to such common indoor flooring materials as vinyl and rubber. Comparisons are made between the theoretical predictions and experimental measurements. Factorial experiments are also designed to study the influence of particle type, flooring type and contact time on particle adhesion force.

Measured and Modeled Charging of a Stratified Chilled Water Thermal Storage Tank with Slotted Pipe Diffusers

As part of ASHRAE Research Project 1185, field data from the constant flow rate charging of a stratified chilled water storage tank with double-ring octagonal slotted-pipe diffusers serving a university chilled water system were compared with results of a transient axisymmetric computational fluid dynamics (CFD) model. Charge processes at flows near design flow rate and 50% of design were modeled. Laminar and turbulent simulations were performed for a range of slot dimensions. Performance of the model was assessed by direct comparison of temperature profiles and by comparison of common thermal performance metrics, including thermocline thickness, lost capacity, and equivalent lost height. The inlet Richardson number based on slot hydraulic radius was found to have the strongest effect on model results. The model Richardson number giving best agreement with the lost height of the field data for near design flow rate was roughly an order of magnitude greater than the Richardson number of the actual diffuser. It is conjectured that this is due to inherent differences between the flow produced by multiple three-dimensional jets in the actual diffuser and the continuous slot of the axisymmetric model. The effect of turbulence varied from negligible to moderate as the Richardson number decreased.

Dimensional Analysis of UVGI Air Disinfection Systems

A dimensional analysis of an ultraviolet germicidal irradiation (UVGI) air disinfection system within a diffusely reflective enclosure is performed using the Buckingham pi theorem. The eight dimensionless parameters obtained include the duct aspect ratio, the lamp aspect ratio, the reflectivity, and terms that incorporate the UV dose, airflow, duct dimensions, and lamp location coordinates. Computer simulation of the dose absorbed by an airborne microbe is used to predict system performance for several thousand combinations of the dimensionless parameters. Statistical analysis of these results is performed to determine the significance of each of the dimensionless parameters and their interactions. Conclusions regarding performance are summarized. The most significant parameters that determine performance of diffusively reflective UVGI systems are shown to be the dose, the reflectivity, the duct geometry, the lamp aspect ratio, lamp location, and various combined functions of these parameters. Practical implications of this research may include the improvement or optimization of UVGI air disinfection systems for in-duct and recirculation unit applications, with a consequent improvement in energy efficiency.

Parametric Study of Single-Pipe Diffusers in Stratified Chilled Water Storage Tanks (RP-1185)

A parametric study was performed of the charging thermal performance of a full-scale pipe diffuser in a single cylindrical stratified chilled water storage tank by applying factorial experimental theory to the results of simulations performed with a validated computational fluid dynamics (CFD) model. Dimensional parameters having the potential to influence charging inlet performance were identified and formed into dimensionless groups using the method of repeating variables. Parameters included: the inlet Richardson number based on inlet slot width (Ri l ), inlet Reynolds number (Re i ), ratio of inlet width to diffuser height (l/hi ), ratio of inlet diffuser height to tank radius (hi/RW ), and ratio of diffuser radius to tank radius (RD/RW ). Thermal performance was measured in terms of equivalent lost tank height (ELH). A full 2k factorial experiment of thirty-two simulations was performed and analyzed. Parameter ranges were: 0.05-2 for Ri l , 500–5000 for Re i , 0.1–1 for l/hi , 0.005–0.05 for hi/RW , and 0.707–0.866 for RD/RW . Within these ranges, Ri l , l/hi , and hi/RW were found to be of first-order significance, while Re i and RD/RW were not. Two-factor interactions involving Ri l , l/hi , and hi/RW were also significant. Regression models of equivalent lost tank height as functions of Ri l , l/hi , and hi/RW were developed. The predictive capabilities of the regression models were tested against the results of five additional CFD simulations having parameter values different from the 2 k factorial experiment cases. On average, regression models predicted factorial experiment data to within 10% with maximum error of 30% to 60%, depending on the model.

Ultraviolet irradiance measurement and modeling for evaluating the effectiveness of in-duct ultraviolet germicidal irradiation devices

This study uses ray-tracing software to calculate the fluence distribution in ultraviolet germicidal irradiation devices with different surface reflectivities and lamp configurations. Of five validation cases considered, one incorporating anisotropic duct surface reflectivity and wind-chill correction of lamp output gave the best agreement with planar irradiance measurements (mean error –3%, standard deviation 9%). The ray-tracing software was used as a validated design tool to evaluate two typical in-duct ultraviolet germicidal irradiation devices (Cases 6 and 7). Four identical UVC lamps were modeled inside a rectangular duct. The lamps were either arranged in parallel or cross flow, and spherical irradiance values along the flow path were compared. Without accounting for the thermal effect on lamp output, an ultraviolet germicidal irradiation device placed in a cross flow would give a higher average UV irradiance. This benefit would be practical when a sufficient straight run is provided in a ventilation system. However, a lamp in parallel flow would produce a more uniform UV irradiance field near the center of the device. Changing the thermal conditions would have significant impact on lamp outputs. Arranging lamps in a parallel flow would provide a higher total irradiance at low temperature and high flow conditions, especially for lamps with outputs lower than the simulated lamps and lamps without sleeves.

Effects of an ultraviolet coil irradiation system on the airside heat transfer coefficient and low ΔT syndrome in a hot and humid climate

Biological fouling (biofouling) on cooling coil surfaces acts as thermal insulation, impeding heat transfer from air to coil surfaces, decreasing airside heat transfer coefficient and degrading coil cooling capacity. It is also a common cause of low ΔT syndrome in chilled water distribution systems. The effects of a commercially available ultraviolet germicidal irradiation system installed in a variable air volume system on the airside heat transfer coefficient, cooling coil capacity, and its potential to mitigate low ΔT syndrome were investigated via a field test. Energy-related measurements including chilled water supply/return temperature, water-/airflow rate and entering/leaving air temperature/relative humidity commenced 4 months before turning on ultraviolet lamps and continued for 10 months after ultraviolet germicidal irradiation intervention. The effects of the ultraviolet germicidal irradiation system were evaluated via a “before ultraviolet” and “after ultraviolet” comparison. After ultraviolet intervention, within the face velocity range of 1.5–3.0 m/s, the airside heat transfer coefficient increased by 11.8%–20.1%, which translated into 8.8%–10.2% increase in the overall enthalpy-based thermal conductance. The coil total cooling capacity and latent cooling capacity increased by 3.3%–3.8% and 4.5%–5.7%, respectively. The chilled water flow rate required to maintain the leaving air temperature set-point decreased by 8.0%–11.9% and the water-side temperature difference increased by 0.4°C–0.6°C.