Analysis of Indirect Evaporative Heat Exchangers: Modeling and Experimentation

Indirect of evaporative cooling (IEC) involves evaporating water to cool the air supplied to a conditioned space without adding moisture. With advantages of lower energy use and improved performance at drier and hotter climates (compared to Direct eXpansion (DX) air conditioners), the IEC is well suited to arid areas for mitigating peak electricity demand that is in large part attributed to the operation of DX air conditioners during summer afternoon hours. Among the components in an IEC-based cooler, the indirect evaporative heat exchanger (IEHX) is the most critical one, as it is the primary source of cooling. Despite great energy saving potential and related benefits, the penetration of IEC-based coolers into DX air conditioner markets is still very low, which, along with other barriers, is partially due to the somewhat low effectiveness of conventional IEHXs and a lack of practical analysis tools to broaden their applicability by HVAC (Heating, Ventilation and Air Conditioning) engineers, both of which are the impetus for this research.;The research presented in this dissertation focused on the IEHX through both modeling and experimental investigations. The modeling work has two components: 1) a general-purpose analytical model that offers a methodology (modified epsilon-NTU method) for thermal analysis of IEHXs, and 2) a simplified CFD model for quickly simulating the thermal-hydraulic performance of a novel IEHX design that has curved (e.g., U-shaped) air flow paths in its narrow channels. The general-purpose analytical model satisfies the requirements of short computational time (seconds), simplified data input, and numerical stability, all of which are desirable for system-level energy simulation of IEC-based coolers; The CFD model computational time is greatly reduced (less than10 seconds using a modern PC desktop) through a depth-averaging technique that reduces the original three-dimensional (3-D) Navier-Stokes (N-S) equations to two-dimensions (2-D) with a unique treatment of the viscous terms. The 2-D N-S equations were further be approximated with a viscous Bernoulli equation. The model was tested for smooth straight and U-shaped narrow channels, all within errors of 10%, as compared, respectively, to analytical and experimental results.;The experimental studies targeted collection of empirical data for pressure drops and water evaporation rates in pin-fin channels that can produce high performance IEHXs. Experiments were conducted in dry conditions (analogous to the IEHX dry channel) and then in wet conditions (analogous to the IEHX wet channel). For dry-condition tests, data on pressure drops in 13 smooth pin-fin channels (staggered, circular pins, 3≤SL/D,ST/D≤9,1.5≤H/D≤3,1464< ReDh<13085) was collected and then correlated into empirical formulas to examine the design variables of pin spacing, diameter, height, and shape (conical v.s. cylindrical). The major findings include: 1) the transverse pin spacing to diameter ratio ST/D has a larger impact on pressure drop than does the longitudinal pin spacing to diameter ratio SL/D; 2) the pin height to diameter ratio H/D has more significant impacts on friction factors for denser pin arrays; and 3) the conical pins produce a 27% friction reduction as compared to circular cylindrical pins of the same mean pin diameter in a smooth rectangular channel of the same aspect ratio. A semi-empirical model derived from physical principles for pressure drop predictions in pin-fin channels (circular pin arrays) was proposed and compared with the 13 smooth pin-fin channel experiments, resulting in an RMS error of 14%, and a maximum error of 20% as hydraulic-diameter Reynolds number ReDh ranged from 4000 to 12000. For wet-condition experiments, data on air-water flow patterns, pressure drops and water evaporation rates (per unit area) in five flocked pin-fin channels (staggered, circular pins, 3≤ SL/D,ST/D≤9,H/D=2,1000Dh<5000, fully wetted flocked surfaces) was collected. The measurements indicate: 1) a low air velocity (less than 5m/s in this study) is favorable in the IEHX wet channel (with air-water counter flow) to avoid flooding; 2) at low channel air velocities (< 5m/s), the impact of the water flow on the friction factors is trivial (<10%) for flocked pin-fin channels in dry and wet conditions (fully wetted surface); 3) measured water evaporation rates increased linearly with channel air velocities; and 4) pin-fin arrays in the IEHX wet channel enhanced water evaporation rates (per unit area) by a factor of 3 to 12 times as compared to channels without pins.