1、Simulation and Experimental Investigation of Condensation in Residential Venting Paul Glanville, PE Larry Brand Shawn Scott Associate Member ASHRAE Member ASHRAE ABSTRACT As high-efficiency gas-fired furnaces and water heaters are retrofitted into existing residential venting systems, the tendency f
2、or corrosive flue gas condensate to form increases in vent connectors, common vents, and masonry chimneys. This issue can arise through a range of scenarios, such as the retrofitting of a condensing furnace in a residence with dedicated venting, “orphaning” an atmospheric gas-fired water heater in t
3、he vent system from which flue gases reach sub-dew point temperatures from cooler vent walls. The cycling of a retrofitted appliance influences condensation dynamics as well, altering the so-called “wet-time” of the flue interior surface, key to its potential for corrosion. Additionally, the vent it
4、self and its operating conditions are important, including the design, proximity to capacity, and ambient conditions are influential. Using the combination of computational tools and a full-scale laboratory exterior masonry chimney, the Gas Technology Institute (GTI) has framed this issue facing res
5、idential venting systems. Much of the work covered in this paper concerns the use and validation of VENT-II, a residential venting simulation software tool for common vented appliances key to the development of the National Fuel Gas Code venting guidelines. Through targeted use of computational flui
6、d dynamics and full-scale experimental testing, GTI has begun an effort to validate and improve the accuracy and validity of the software, initially focusing on the performance of hot water boilers installed in exterior masonry chimneys. Through this validation, GTI has studied the impact of retrofi
7、t scenarios for vent system designs that are on the margins of compliance with the National Fuel Gas Code. As residences approach Zero Energy Designs, the push for higher efficiency appliances will continue to present challenges to safe and efficient venting systems. INTRODUCTION A major market barr
8、ier to upgrading the efficiency of or fuel-switching with a heating system is the cost associated with the chimney liner and its installation. According to one utilitys cost breakdown, the chimney liner constitutes 25% of the total conversion cost of fuel-switching. This conversion is subject to pro
9、visions of both NFPA 54 National Fuel Gas Code (NFGC) and NFPA 211 Standard for Chimneys, Fireplaces, Vents and Solid Fuel Burning Appliances. This is, however, in contrast to the costs brought upon by pitting corrosion, vent failure, and potential for unsafe venting of products of combustion (DeWer
10、th, 1983). With respect to pertinent codes, it is the responsibility of both the regulators and regulated community to periodically re-examine the underlying assumptions and technical process behind standing code guidance. The research community may aid in the process, as with any unresolved technic
11、al issues that arise in that applicable code and installation practice evaluation can be addressed by modeling and possibly laboratory investigation to justify those code compliance interpretations. In the current study, the project team conducted a technical evaluation and laboratory investigation
12、of factors affecting LV-11-C012 2011 ASHRAE 952011. American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions, Volume 117, Part 1. For personal use only. Additional reproduction, distribution, or transmission in either print or
13、 digital form is not permitted without ASHRAES prior written permission.venting requirements for gas vent systems in a representative utility service territory fuel-switching, thus vent system upgrades, are more common, the U.S. Northeast. Focusing on the potential for relining requirements during t
14、hese upgrades, this study considers venting systems with masonry chimneys. Hydronic (hot water) boilers are prevalent in this region, which were treated as equivalent to furnaces in the original development of the NFGC (Philips, 1994): “The differences between a boiler and a furnace for the purposes
15、 of a vent system analysis are twofold: The cycle rate at 50-percent load is 2 cycles per hour for a boiler (this directly affects furnace on-times as well), and A boiler is typically equipped with a draft hood and a stack damper. Based upon these results, there is no significant difference between
16、a boiler and a furnace in terms of exterior masonry chimney wet-times predicted by VENT-II (for the same steady-state efficiency).” Higher efficient gas-fired appliances in Category I venting systems are more likely to result in condensation within exterior masonry chimneys. The use of vent dampers
17、with hot water boilers reduces the off-cycle flow significantly, reducing the ability of the vent system to dry out between on-cycles. Additionally, boilers with vent dampers have different off-cycle flow characteristics than the furnaces used to develop the Category I appliance venting tables in th
18、e NFGC (hereafter referred to as “venting tables”). Performance is assessed primarily through numerical modeling, both using VENT-II and Computational Fluid Dynamics (CFD). VENT-II software was developed for the Gas Research Institute (GRI) during the original development of the venting tables (DeWe
19、rth, 1983). FLUENT version 6.3 is used for CFD, modeling fluid flow, heat transfer, and phase change. As VENT-II is a simpler 1-D modeling tool with a Graphical User Interface (GUI), it has been a useful tool in venting research. With its finer resolution and more sophisticated algorithms, CFD is us
20、ed to assess the relative veracity of VENT-II in modeling venting systems for varied boiler efficiency, firing rate, ambient condition, number of exposed chimney walls, and presence of chimney liner. Simulation results are compared with experimental data from a full-scale test masonry chimney, using
21、 the metric Boolean continuously “wet” or “dry” conditions as used in the NFGC. MODEL DEVELOPMENT AND ASSUMPTIONS The objective in using CFD modeling is to use a sophisticated tool selected both to compare to VENT-II simulation results and experimental data, while exploring the venting dynamics of g
22、as-fired boilers versus relative to that of fan-assisted furnaces, the subject of prior study and the venting tables. VENT-II is a one dimensional nodal model, solving a reduced form of the Navier-Stokes equations and a semi-empirical condensation model at the interior flue surface. In addition to c
23、ycling and ambient conditions, boundary conditions at the appliance outlets characterize flue gas properties relative to user-defined cycling, temperature profiles, firing rates, and percentage of excess air. FLUENT version 6.3 is the CFD simulation software used, with turbulence modeled using the k
24、- model. The fluid domain includes the chimney interior and the ambient surroundings, primarily included for solution stability, and the solid domain is the chimney itself for heat transfer modeling. Sensitivity of condensation results to meshing of the chimney interior was explored, which is shown
25、in Figure 1 in addition to this computational domain. Each simulation and chimney configuration utilized this mesh. Initial screening CFD results that do not directly model condensation show order of magnitude agreements with the VENT-II analysis, there are known gaps in modeling physics concerning
26、the mass and heat transfer associated with condensation. The source code of FLUENT is not accessible for modification; however it offers many so-called access points where users may program additional physical models to be included with those built-in. This is how condensation is directly modeled, w
27、ith customizable physical models. To directly model surface condensation in CFD, a so-called User Defined Function (UDF) is programmed which models condensation as a volumetric reaction rate coupled with the latent energy source/sink. To prevent numerical instabilities and solution divergence in the
28、 implementation of a UDF during CFD simulation, a simplified approach is taken to the UDF formulation. In brief, the UDF is defined as follows: 1. A volumetric reaction is defined through macros in the source code, which is preferential over a surface reaction to minimize the numerical instability c
29、aused by large intra-cell gradients. As water vapor is a small fraction of the overall 96 ASHRAE Transactionsflue gas and the fraction that condenses is even smaller, gradients from cell face to cell face will be very large, causing solver oscillations. 2. Reviewing similarly employed condensation m
30、odels in the literature, they fit into one of three categories in increasing complexity: simple mass flux, mass diffusion via an empirical diffusivity, and heat diffusion based upon film or drop-wise condensation on various geometries. After performing extensive modeling of flue gas condensation, Pe
31、rujo et al. (2004) found that for a sufficiently fine mesh, the mass flux and mass diffusion models yield similar results within 10% of one another, which due to its simplicity was selected (Perujo, 2004). This model states conservatively that water vapor in excess of the saturated cell condition wi
32、ll condense. This is determined after calculating the water vapor saturation pressure at each cell (ASHRAE 2009), the actual and saturated water vapor concentration is determined. 3. Proportional to the water vapor mass condensing, there is a latent heat release for that cell. Figure 1 Exterior Maso
33、nry Chimney CFD Domain (left), Graphic of Horizontal Chimney Cross-Section (center), and Example of Graphical Output for Contours of Relative Humidity (right) In the baselining of this condensation modeling methodology, a non-compliant common venting of a fan-assisted furnace and natural-draft gas-f
34、ired water heater into both interior and exterior masonry chimneys is simulated with VENT-II and CFD. Comparing these simulation results, the interior and exterior masonry chimney simulations result in net condensation gains within the chimney segment above the roofline within 43% and 22% of each ot
35、her (with CFD showing consistent overestimation), showing order of magnitude agreement. As the primary criteria of simulation-based venting table development was the Boolean “wet” or “not wet”, this agreement is sufficient, and this modeling method was adopted. Simulation Strategy however the off-cy
36、cle flow is 15% of the on-cycle flow, which is insufficient to pick up the accumulated condensate. This results in a net gain of condensate and continuously wet conditions. These results qualitatively match the VENT-II simulation of a boiler equipped with a vent damper reducing off-cycle flow to 15%
37、 of on-cycle flow. 100 ASHRAE TransactionsFigure 3 Photos of Laboratory Chimney Construction Comparing the on-cycle mid-chimney surface temperatures of another 13 hour test with an average outdoor temperature of 25F (-4C) with the previous transient CFD modeling results of Case 13, Figure 4, shows g
38、ood agreement. As off-cycle flow modeled in Case 13 was larger, by approximately a factor of 2.8, the heat capacity of the off-cycle flow is increased, in part explaining the discrepancy in off-cycle temperatures. The relative agreement in chimney surface temperatures during the on-cycle, despite a
39、difference of maximum boiler flue gas exit temperatures of over 40 F (4C), highlights the importance in off-cycle flow with respect to conditions within the chimney. Figure 4 Experimental vs. CFD Data for Flue Gas Temperature at Liner Surface at Mid-Elevation CFD modeling of Case 11, for an exterior
40、 masonry chimney with three sides exposed, a clay tile liner, outdoor temperature of 13 F (-10.5C), and an 83 % efficient boiler; reported dry conditions for the transient simulation and near negligible condensation for the steady state simulation, despite anecdotal expectations of continuously wet
41、conditions. Three full 13 hour tests were performed, with photographs in Figure 5, which were different combinations tests with or without common venting with water heater and equipping the boiler with a vent damper: (1) both, (2) no water heater, and (3) no vent damper. Cases 1 and 2 were continuou
42、sly wet, highlighting the importance of off-cycle flow magnitude. Prior expectations were likely based upon observations of boilers with a vent damper, reducing the off-cycle flow rate close to that previously measured 0.6 lb/min (0.3 kg/min). The initial CFD modeling of Case 11, like all cases simu
43、lated, used an estimate of 40% off-cycle flow consistent with the ASHRAE 103 & ANSI Z21.47 AFUE test and surveys of manufacturers and installers, covering the breadth of boiler system pressure drops. As measured, test 3 without a vent damper had an off-cycle flow of approximately 90% of on-cycle flo
44、w, compared to 40% and 15% of CFD run for Case 11 and test 1 respectively. 2011 ASHRAE 101Figure 5 Photos of Chimney Exit Following Full Cycle Tests 1, 2, and 3 (Left to Right) CONCLUSION Historically, the NFGC venting tables were concerned with furnaces with respect to chimney and vent connector co
45、ndensation. The development focused on fan-assisted furnaces, as modeled during the venting table development, which were not equipped with vent dampers. Boilers were determined to have an equivalent impact on venting systems with respect to the potential for in-flue condensation, despite difference
46、s in cycling, flue gas dew point temperatures, and presence of vent dampers. As boilers are prevalent in the U.S. Northeast, the focus of this investigation into the veracity of this assertion is with exterior masonry chimneys, while performing a comparison of two simulation tools, VENT-II and CFD.
47、Following a direct comparison of results, the underlying physical models of VENT-II provide sufficiently accurate predictions of potential for condensation as compared to CFD modeling. VENT-II, CFD, and laboratory experimentation confirm the relining recommendations in the NFGC venting tables for th
48、e cases studied, using assumptions from their original development. From this study, the primary factors influencing condensation within exterior masonry chimneys identified are the appliance efficiencies and magnitude of off-cycle flow from hot water boilers. With respect to causing continuously we
49、t conditions, higher efficiencies reduce flue gas dew point temperatures, leading to condensing during the on-cycle, and a reduced off-cycle flow limits the drying of the chimney interior. In this study, test data of a hot water boiler and natural draft water heater venting common vented into an exterior masonry laboratory chimney, and subsequent model validation, have highlighted the relative importance of off-cycle flow magnitude over the appliance efficiency. Exploring this issue followed a discrepancy between Simulation Case 11 and expectations of continuously wet
