Conference Agenda

When moist air passes through a cooling coil, the potential exists for that moisture to condense on the coil's surface. The temperature at which that occurs is the condensing temperature or the dew point temperature. The dew-point temperature is determined by air pressure and humidity based on thermodynamic theory. The condensate – condensed water vapor – usually remains on the cold surface for some amount of time before being removed by gravity, the air stream, or both. The temperature of that condensate may not be the same as the dew-point temperature. In formulas found in the 2020 ASHRAE handbook – “HVAC Systems and Equipment,” condensate temperature is used as the wet-bulb temperature at the leaving condition. In this study, a 12-row cooling coil was tested under fully wetted surface conditions. The condensate temperature profile along the air-flow direction as well as the averaged condensate temperature were measured under different inlet humidities and face velocities. The test results showed that the condensate temperature was higher than the wet-bulb temperature. Factors that contributed to this included air temperature, inlet humidity, air velocity, water temperature, and water velocity. When moist air passes through a cooling coil, the potential exists for that moisture to condense on the coil's surface. The temperature at which that occurs is the condensing temperature or the dew point temperature. The dew-point temperature is determined by air pressure and humidity based on thermodynamic theory. The condensate – condensed water vapor – usually remains on the cold surface for some amount of time before being removed by gravity, the air stream, or both. The temperature of that condensate may not be the same as the dew-point temperature. In formulas found in the 2020 ASHRAE handbook – “HVAC Systems and Equipment,” condensate temperature is used as the wet-bulb temperature at the leaving condition. In this study, a 12-row cooling coil was tested under fully wetted surface conditions. The condensate temperature profile along the air-flow direction as well as the averaged condensate temperature were measured under different inlet humidities and face velocities. The test results showed that the condensate temperature was higher than the wet-bulb temperature. Factors that contributed to this included air temperature, inlet humidity, air velocity, water temperature, and water velocity.

Standard coil performance testing is conducted so that air flow is perpendicular to the coil’s front surface. The air friction values learned from that test are then used to inform fan selection. But in many mid-sized and large HAVC systems, coils oriented in a V-Shaped Coil bank configuration are very popular. In such an orientation, the air flow direction relative to the front surface of the coil may change, which could impact the air friction values in the application. In this study, the air friction from V-Shaped coil bank was measured at a range of face velocities (100 to 1000 FPM) and the angle forming the V-Shape was measured from 15 to 60 deg. The results were also compared with those of a slab coil. In general, the higher face velocity caused higher air friction, and more acute V-Shaped angles resulted in higher air friction. These measurement results will help improve the accuracy of fan selections in HVAC systems that feature V-shaped coil bank.

Water bridge formation between fins can occur when the heat transfer surface temperature is lower than the dew point, and it can also occur when the ice melts during the defrosting/deicing process. The water bridge can lead to a significant drop in the thermal and hydraulic performance of a heat exchanger. Predicting water bridging is important for estimating the heat exchanger performance under both wet/condensation conditions and frosting/defrosting conditions.

In this work, experiments on small-scale fin arrays have been conducted to observe condensate droplets sliding and water bridging processes using high-speed cameras. The droplet shape evolvement and coalescence/bridging characteristics will be captured and quantified. Effects of different factors, including fin dimensions, fin gap, airflow conditions, will be presented and discussed. Models to characterize the geometric parameters of the retaining condensate droplets on the fin surface have been developed based on the experimental observation. Additionally, hydrophilic-hydrophobic wettability gradient is introduced to the fin surfaces, and the impact of hydrophobic coating length has been evaluated. The results derived from this work can provide guidance for the heat exchanger fin design to reduce water bridges and optimize thermal-hydraulic performance.

In this study, experiments were conducted on a fin-and-tube heat exchanger under both wet and dry conditions. Throughout the experiments, the air inlet temperature to the heat exchanger remained constant. Tests were carried out at relative humidities of 30%, 70%, 80%, and 90% for both dry and wet conditions. Additionally, experiments were conducted at five different air velocities for each relative humidity. Since the same heat exchanger was used for all test conditions, the geometric parameters of the heat exchanger remained unchanged during the experiments. The aim of this study is to investigate the performance of a finned-tube heat exchanger under different tube arrangements in both dry and wet conditions, a topic that has not been extensively covered in the literature. The experimental data were analyzed theoretically, first comparing the heat transfer coefficient and then the Colburn J-factor. These analyses were compared with some commonly used correlations in the literature. Based on the results obtained from the experiments, a comparison was made regarding the air-side performance of the finned-tube heat exchanger. The pressure drop values on the air side under wet conditions showed similar results at the same Reynolds number, despite varying relative humidities. There was a difference of 3% to 10% in the Reynolds number under wet conditions. When comparing the pressure drop values on the air side between dry and wet conditions at the same Reynolds number, there was an average difference of 71%.The Colburn j-factor and heat transfer coefficient results are consistent with each other. The heat transfer coefficient values under dry and wet conditions were approximately 2.5 times higher at low relative humidities, increasing up to 3.1 times at 90% relative humidity

With the exponential rise in the amount of data being processed, stored, and transmitted in service of modern computing applications including large language model training, high-bandwidth video processing, and others, data centers currently consume 2% of electricity generated in the U.S. alone, with expected increasing levels of energy use in the next decade. Aside from the energy used by the electronic devices themselves, the primary parasitic energy use in these facilities is for thermal management. In this context, pumped two-phase loops are being investigated to provide effective and efficient cooling for data centers. Leveraging the high heat transfer capabilities achieved in boiling and condensation, pumped two-phase loop systems can address the cooling demands of high-power density chips more effectively than conventional single-phase air- or liquid-cooling technologies, thereby eliminating the use of energy intensive chilled water or refrigerated air as coolants. Strict space requirements are placed on the cold plates that reside in the server racks of these systems, leading to extensive study. Comparatively, condensers have been less extensively researched, even though they play a critical role in dissipating heat out of the cooling cycle. Therefore, condensers are often oversized to overcome sub-optimal performance caused by, for example, maldistribution. Microchannel heat exchangers are often selected as the condenser geometries in these systems due to their compact design. Although condensation in these geometries has been widely studied for traditional HVAC applications, the unique boundary and operating conditions of electronics cooling applications (namely, low inlet vapor quality, distinctive desired saturation temperatures of the working fluid, non-traditional orientation installation into server racks, etc.) necessitate a more application-specific investigation into their condensing phenomena.

In this study, an experimental condenser test stand has been designed and constructed that emulates pumped two-phase systems. An air-cooled microchannel condenser with channel hydraulic diameter of 0.64 mm was tested with R1234ze(E), a low-GWP dielectric fluid. Inlet vapor quality was varied between 0.2 and 1.0 over a range of saturation temperatures between 30 and 60 °C and mass fluxes between 40 and 100 kg/m²-s. Experiments were conducted on both vertical and horizontal orientations of the condenser. The capabilities of the facility demonstrated in this study can aid in the proper validation of a numerical model to predict condenser behavior specific to this application.

TriCoil^TM is a novel three-fluid fin and tube heat exchanger that enables integration of thermal energy storage (TES) with residential heating, ventilation, and air conditioning (HVAC) systems. A unique feature of TriCoil^TM is that it allows direct refrigerant-to-air cooling and heating operation. This study compares the performance of three new circuit arrangements with the first-generation design. We used a validated model of TriCoil^TM, based on OSU CIBS’s discretized xFin model, to explore circuit arrangements of the coil for cooling applications. Coil capacity and thermohydraulic effectiveness on the water side are considered as the primary performance parameters. Initial findings suggest that a specific circuit arrangement can reduce water-side pressure drop by a considerable amount without any significant loss in heat exchanger capacity. Furthermore, it is possible to surpass the coil hydronic operation capacity of the previous design with identical pumping power. We assess system-level application through parametric study of latent capacity in continuous discharge operation. Here, we also present the differences in heat exchanger performance for direct (refrigerant-to-air) cooling operation.