Non-uniform Airflow Distribution Research Articles

Optimization study of the circuitry for 5 mm small-diameter finned-tube heat exchanger in air-source heat pumps

Finned-tube heat exchangers (FTHXs) are currently extensively utilized in residential air-source heat pumps (ASHPs), but their escalating costs pose an urgent issue needing resolution. This study investigated the feasibility of replacing the 7 mm diameter FTHX in the outdoor unit of an ASHP with a 5 mm small-diameter FTHX, aiming to reduce material consumption without compromising performance. Based on the Intelligent System for Heat Exchanger Design (ISHED), a 6–3-1 circuitry (6 circuitries merging into 3 then 1) optimization scheme tailored for the 5 mm diameter FTHX was proposed under non-uniform airflow distribution. The performance differences between this circuitry and the baseline were then analyzed through numerical simulations. Finally, comparative tests were performed between the original FTHX and the modified 5 mm diameter FTHX. Overall, even with a 5.10 % reduction in total heat exchange area, its heat exchange capacity and efficiency still meet the performance standards for the initial ASHP. Notably, the proposed 5 mm diameter FTHX reduces the material costs and refrigerant charge by 19.76 % and 12.28 %, respectively. This phenomenon illustrates the considerable economic benefits of reducing the volume of FTHX. Additionally, it holds significant practical value in promoting the use of natural refrigerants including hydrocarbons according to the new international legislative framework.

Frost formation non-uniformity in a multi-circuit V-shape evaporator

Formation of frost on outdoor evaporators poses a significant challenge for air source heat pumps operating in cold climates. It is widely recognized that uneven air-flow across an evaporator can have a negative impact on both the evaporator’s performance and the overall efficiency of the heat pump. This study investigates the influence of non-uniform air-flow distribution on a V-shaped outdoor evaporator as a component of an air-source transcritical CO2 heat pump when subjected to frosting conditions. To this end, we developed and validated against independent experiments a numerical frost-formation model, and used it to study a multi-circuit evaporator of interest under different ambient conditions. It is shown that the uneven air-flow distribution results in varying frost formation patterns across different circuits of the heat exchanger, including differences in the amount of frost accumulated and its distribution among the individual tubes; the circuit with the largest frost weight accumulates 18% more frost compared to the one with the smallest in one hour under the sever frosting condition. Non-uniform frost formation also leads to air-flow distribution changing over time, and the rate at which this change occurs depends on the ambient conditions. For the presently investigated case, frost formation suppresses the non-uniformity of air-flow distribution. Moreover, evaporators with non-uniform and ideal uniform air-flows are compared and it is revealed that the difference in their capacities is more noticeable at the beginning of the frosting period.

Performance of upper-room ultraviolet germicidal irradiation (UVGI) system in learning environments: Effects of ventilation rate, UV fluence rate, and UV radiating volume

• We study performance of upper-room ultraviolet germicidal irradiation (UVGI) systems. • Performance of upper-room UVGI system in a classroom varies with ventilation rate. • Non-uniform airflow pattern meaningfully influences the UVGI system performance. • UV radiating volume is more crucial for virus disinfection than UV fluence rate. • Doubling UV fluence rate from 25 to 50 μW∙cm-2 yields only moderate benefit. Previous studies show that upper-room ultraviolet germicidal irradiation (UVGI) systems can help contain infectious airborne viruses indoors. However, there has been a lack of research on the performance of an upper-room UVGI system in learning environments such as a school classroom. Since classrooms are more vulnerable to airborne transmission of diseases due to high occupancy for long hours, airborne infection characteristics are different from other occupied indoor environments (e.g., offices and residences). The objective of this study is to investigate UVGI system performance in a classroom considering detailed effects of ventilation rate, UV fluence rate, and UV radiating volume. Two analytical models, a one-zone and a two-zone material balance model, along with computational fluid dynamics (CFD) simulations, were employed to analyze viral aerosol concentrations under a representative range of classroom operating conditions. The CFD results show that increasing ventilation rate from 1.1 h –1 to 5 h –1 yields about 85% of airborne disinfection while doubling UV radiating volume results in a 60% disinfection. However, increasing UV fluence rate from 25 μW∙cm –2 to 50 μW∙cm –2 yields a moderate additional disinfection of 18%. Overall, the study results reveal that operating a UVGI system in an occupied classroom can markedly disinfect airborne viruses up to 96%, which is as effective as increasing ventilation rate more than five times. Furthermore, the results suggest that the one-zone and two-zone analytical models used in several previous studies could result in notably meaningful errors in analyzing viral aerosol concentrations, especially in occupied rooms with a highly non-uniform airflow distribution.

Dynamic moist air monitor in a micro area with extremely high figure-of-merit.

In the rapidly changing moisture air, conventional relative humidity (RH) sensors are often difficult to respond in time and accurately due to the limitation of flow rate and non-uniform airflow distribution. In this study, we numerically demonstrate that humidity changes on micro-zones can be monitored in real time using a Bloch surface wave (BSW) ubiquitous in one-dimensional photonic crystals (1DPC). This phenomenon can be observed by leakage radiation microscope (LRM). After theoretically deriving the angular resolution limit of LRM, we obtained the minimum BSW angular change on a practical scheme that can be observed in the momentum space to complete the detection, and realized the dynamic real-time monitoring of small-scale humidity change in experiment for the first time. This monitoring method has extremely high figure of merit (FOM) without hysteresis, which can be used in humidity sensing and refractive index sensing as well as the research on turbulence.

Numerical modeling of an outdoor unit heat exchanger for residential heat pump systems with nonuniform airflow and refrigerant distribution

Multirow finned tube heat exchangers have been extensively used as an outdoor unit in heat pump applications. Several factors, including nonuniform airflow distribution, refrigerant circuitry arrangement, fin and tube configurations, and operating conditions, have a significant influence on its thermohydraulic performance. For accurate outdoor unit heat exchanger modeling and analysis, developing a robust and experimentally validated numerical model reflecting the influence of realistic nonuniform airflow and refrigerant distribution is necessary. This paper presents a general-purpose numerical model, implemented in MATLAB, for a multirow finned tube heat exchanger using a tube-by-tube modeling approach based on the log mean enthalpy difference method. State-of-the-art heat transfer and pressure drop correlations are used for each refrigerant-side's flow regime and airside fin configuration. An alternative iteration loop is also incorporated to solve the complex circuitry of the heat exchanger under wet conditions. Simulation results of the current model yield consistent and more real heat transfer, pressure drop, and temperature contours using nonuniform airflow velocity profiles than those of the simplified models that assumed uniform airflow. Therefore, higher accuracy of the performance evaluation for complex refrigerant circuitry was achieved. Furthermore, current model is able to identify the exit vapor quality that allows proper selection of tubes and mass flow rates to avoid unnecessary superheat or sub cooling. The model is also validated with available experimental data, and the maximum error is within ±10.0%.

Prototype energy models for data centers

Data centers in the United States consume about twopercent of the nation’s electricity. Because heat gains from IT equipment drive cooling demand, data centers offer unique opportunities for energy savings. However, no prototype energy model for data centers is available in the suite of existing U.S. Department of Energy’s Commercial Prototype Building Models. This study presented the development of two new data center prototype models and their implementation in OpenStudio and EnergyPlus. The small-size data center model represents a computer room in a building served by computer room air conditioners (CRACs); while the large-sized model represents stand-alone data centers served by computer room air handlers (CRAHs) with a central chiller plant. For each data center model, two levels of IT equipment (ITE) load density were considered, to cover the wide range of IT power density of data centers: 40 and 100 W/ft2 (430 and 1076 W/m2) for the computer room, and 100 and 500 W/ft2 (1076 and 5382 W/m2) for the stand-alone data center. All other assumptions, such as building envelope, lighting, HVAC efficiencies and schedules, were based on the minimal requirements of ASHRAE Standard 90.1 at various vintages. We introduced a novel concept of supply and return air approach temperatures to capture the essential effects of non-uniform airflow and temperature distribution in data centers. The approach temperatures were pre-computed by computational fluid dynamics (CFD) simulations for various configurations of ITE loads and airflow containment management in data centers. A new feature was developed in EnergyPlus to implement the approach temperature method. A case study was conducted to demonstrate the use of the data center models. The two data center models cover all U.S. climate zones and can be used to evaluate energy saving measures for data centers, as well as to support development of data center energy efficiency codes and standards.

A novel method integrating windbreak walls with water distribution to mitigate the crosswind effects on natural draft dry cooling towers

This paper proposed a novel method combining both air side optimization (windbreak walls) and water side optimization (water distribution) to reverse the side effects of crosswind on natural draft dry cooling towers (NDDCTs). A three dimensional (3-D) numerical model of a 20 ​m NDDCT based on the University of Queensland NDDCT Gatton test rig is developed. Windbreak walls, designed at three different attack angles towards wind direction (α ​= ​0°, 30° and 60°), could generate an extra amount of the cooling capacity especially at high crosswind speeds, but the contribution is sensitive to the wind attack angles. While water distribution to improve the cooling performance of Gatton NDDCT is not as good as that brought by windbreak walls. However, water distribution could make use of the non-uniform air flow distribution caused by windbreak walls, exploring much more cooling capacity under windy conditions, therefore. With the favor of water distribution, the cooling performance of Gatton NDDCT with walls could be further lifted by 1.37%, 3.24%, 5.75%, 9.66%, 13.07% and 16.06%, respectively from vcw ​= ​3 ​m/s to 8 ​m/s. In summary, the cooling performance of Gatton NDDCT can be lifted beyond or almost maintained at the designed value under all crosswind speeds and wind attack angles.

Approximated flow characteristics of multi-pipe earth-to-air heat exchangers for thermal analysis under variable airflow conditions

Abstract The design process of multi-pipe earth-to-air heat exchangers requires mainly two considerations: the calculations of (i) thermal and (ii) flow performance. The influence of non-uniform airflow distribution among branch-pipes on the thermal performance is usually neglected, although it does occur in multi-pipe structures. In order to bridge this knowledge gap, in this paper, the formulas which enable calculation of an exchanger’s total pressure losses and individual branch-pipe airflows as a function of total airflow are presented. The results show that pressure losses for 45° structures can be up to 30% lower than those for 90° structures, and the maximum airflow in a single branch-pipe can be more than 10 times higher than the minimum airflow measured in another branch-pipe. To demonstrate the influence of the non-uniform airflow distribution between parallel branch-pipes on the heat exchanger’s thermal performance, an example analysis was carried out. The results show that heat and cool gains calculated over one year for real airflows (derived from the approximated flow characteristics of the exchangers presented in this paper) can be up to 20% lower than the maximum possible gains calculated assuming ideally uniform airflow distribution between parallel branch-pipes.

Computational fluid dynamics modelling of the visible light photocatalytic oxidation process of toluene for indoor building materials with locally doped titanium dioxide

Computational fluid dynamics (CFD) is one of the promising methods that can precisely predict non-uniform air flow and contaminant distribution in indoor environments. The overarching objective of this study was to develop a mathematical model for describing the photocatalytic oxidation (PCO) reaction mechanism of gas phase toluene with titanium dioxide (TiO2)-bound indoor building materials. This mathematical model was developed based on Langmuir-Hinshelwood type kinetics and for the integration with CFD simulations as a wall surface boundary condition. The effects of gas phase toluene concentration, illuminance and humidity on the toluene oxidation reaction were considered with locally TiO2-doped building materials. Especially, humidity dependence was explicitly integrated as a competitive adsorption model between toluene and water vapour. Moreover, surface compositions of TiO2 and the substrate (ceramic tile in this study), and the physical adsorption properties of those materials, were modelled and integrated into the mathematical model. A 0.02 m3 chamber experiment and adsorption isotherm measurements were conducted to identify the model parameters. CFD analysis was carried out according to experimental scenarios, and an optimization procedure for the model parameters was proposed for their application as the boundary conditions in the CFD analysis.

Effects of air supply operation on the distribution and spatial differences of airflow field in conditioned room: A CFD Study

In air-blast freezer, there are differences in food freezing process at different locations due to the non-uniform airflow distribution in freezing chamber. In this paper, a decoupling method was used to simulate freezing process of food products at different locations. Firstly, the internal airflow distribution model of the freezing chamber was established to simulate the airflow field in freezing chamber under different velocities of air supply. The mainstream air flow rate around each food product was then calculated as the ambient air velocity at which the food freezes. Then, a validated lumped parameter model was used to simulate the freezing process of individual food product, and the effects of freezing conditions on freezing process at different locations were studied. The results showed that there were large differences in the freezing conditions of foods at different locations in freezing chamber, which were mainly reflected by the difference in ambient air velocity. Compared to air velocity, air temperature was distributed rather uniformly in freezing chamber. With the increase of supply air velocity of air coolers, the ambient air velocity around food products in freezing chamber increased almost linearly.

Prediction of the frost growth and performance change of air source heat pump system under various frosting conditions

A numerical model of air source heat pump (ASHP) system under frosting conditions is developed to predict the frost growth rate and performance change of the heat pump system under various frosting conditions. The modified ε-NTU model is introduced to compute the heat transfer rate of the evaporator with latent heat generation from the water vapor sublimation. Local frost growth is considered to obtain the non-uniform airflow distribution, air pressure drop, and heat transfer rate. The effect of the frost deposition on the ASHP system is investigated from the suggested model under various frosting conditions. It is shown that the performance drop mostly occurs in the latter part of the frosting process, and the mass of the deposited frost is not directly related to the performance drop. The experimental set up is set to verify the proposed simulation model. The simulation results are in good agreement with the experimental data.

CFD modeling of air flow distribution in rice bin storage system with different grain mass configurations

Poor airflow distribution in grain mass during in-bin aeration, drying and storage may lead to moisture content variations that could be detrimental to grain quality. The effects of grain mass configuration and porosity on airflow distribution inside a rice bin were investigated using three-dimensional computational fluid dynamics simulations and experiments. A finite volume method with porous media formulation was used to simulate air flow characteristics in peaked, inverted, and levelled grain mass configurations for long-grain rough rice with a porosity of 0.55, and mean particle size distribution of 2.94 mm. The airflows through the rough rice masses were simulated for airflow rates of 0.55, 0.825 and 1.1 m 3 min −1 [air] t −1 [rice]. The model was validated using a bench scale pressure drop system and an actual long-grain rice in-bin storage with peaked grain mass configuration having a capacity of 700 Mt. The results showed that long-grain rice has viscous and inertial resistance coefficients of 9.72E+06 and 36,185, respectively. Non-uniform airflow distribution dominated peaked and inverted grain mass configurations with peaked configuration having the highest restriction to airflow. Airflow at peak positions in the bed were significantly (p

3D CFD study of the effect of inlet air flow maldistribution on plate-fin-tube heat exchanger design and thermal–hydraulic performance

Plate fin-and-tube heat exchangers are used extensively in heating, ventilating, and air conditioning as well as in refrigeration systems. Non-uniform inlet air flow distribution has a substantial effect on heat exchanger performance. Building upon and extending the work of Yaïci et al. (2014), the present study utilises three-dimensional (3D) Computational Fluid Dynamics (CFD) simulations in order to predict the effect of inlet air flow maldistribution on the design and thermal–hydraulic performance of heat exchangers used in air handling units. Computational assessments of the air-side heat transfer and pressure drop characteristics for a variety of inlet air flow distributions and geometrical parameters of staggered plate fin-and-tube heat exchangers were undertaken on heat exchangers with different longitudinal tube pitches, transversal tube pitches, and fin pitches. Characteristics of predicted thermal–hydraulic performance were reported in terms of Colburn j-factor, Fanning friction f-factor, and efficiency index j/f as a function of Reynolds (Re) numbers. The 3D CFD simulations reveal that air flow maldistribution and the effects of geometrical parameters significantly impact the design and performance of heat exchangers. When compared to baseline (i.e., a uniform inlet air velocity profile), substantial fluctuations in the j- and f-factors—deviations as pronounced as 50–70%, 60–66%, and 63–67% for cases with longitudinal tube pitch, transversal tube pitch, and fin pitch variations with non-uniform air flows, respectively—were observed. The 3D CFD method employed by this study has great potential for use in, first, assessments of correlations centred on air flow maldistribution and, second, efforts to optimally design the headers of heat exchangers in order to minimise inlet flow maldistribution and maximise overall system performance.

Numerical investigations on the effects of different cooling periods in reflow-soldering process

Cooling periods in the reflow soldering process significantly influence heat transfer coefficient and temperature distribution on solder joints, which constituently contribute to soldering defects. The domain of a desktop lead-free reflow oven was built based on the actual dimensions of a sample in GAMBIT 2.3.16. The model was exported to FLUENT 6.3.26 for further parametric study. After grid independency test, simulation results were validated by experimental data according to the American Society of Mechanical Engineers standard for computational fluid dynamics and heat transfer. Next, a simulation model was used to analyze the correlation between temperatures, heat transfer coefficient, and heat flux against reflow cooling durations. Results revealed that a very high radiation heat flux (122.760 W/m2) in a short cooling duration result in unmelted solder powders, which contribute to poor wetting. However, the reduced heat flux of 9.262 W/m2 affects the mechanical properties of solder balls with its continued cooling. Temperature variation and heat transfer coefficient on different points of the substrate are attributed to the non-uniform airflow distribution during the cooling stage. To sum up, the numerical investigation correlates temperature, heat transfer coefficient, and heat flux distribution on different reflow cooling periods to determine their effect on soldering quality.

Investigation of Air Flow Distribution in Small-Scale Food Products Dryer

Experimental and numerical investigations showed that the air flow velocity distribution between the shelves with identical inlet cross sections is nonuniform; in the gaps between the shelves located in the bottom part of the dryer the velocity is rather high and in the gaps between the shelves located in the middle and at the top of the dryer the velocity is low. A nonuniform air flow distribution causes unequal drying of the product; therefore, the position of shelves with the products must be periodically changed; that is, operation of the dryer is not optimal. It was revealed during numerical investigations that the use of variable inlet cross sections between the shelves gives more uniform air flow distribution. Numerically obtained air flow velocity distribution results between the shelves with the use of variable inlet cross sections were also justified experimentally.

Three-dimensional airflow distribution in a maize silo with peaked, levelled and cored grain mass configurations

Non-uniform airflow models for peaked, levelled and cored grain mass configurations in a maize silo were developed using the finite volume method. Airflow resistance due to porous media of grain material was implemented using Ergun’s equation. The linear porosity variation was implemented with low porosity at the centre and maximum at the side. Non-uniform airflow distribution for natural air drying with an airflow rate of 1.1 m3 min−1 t−1 was validated with experimental data using maize in grain bins with storage capacities of 180.0–450.0 Mt. Air velocity was predicted with reasonable accuracy for cored, levelled and peaked grain mass configurations having porosities ranging from 0.34 in the core to 0.38 towards the silo wall. Incorporation of variable porosities into the model improved the air velocity prediction. For the cored grain mass and variable porosity (0.34–0.38), the error was 4.4% at the centre and 23.1% at the periphery.

Effect of the Moments of Probability Density Function for Non-uniform Air Flow Distribution on the Hydraulic Performance of a Fin-tube Heat Exchanger

Effect of the Moments of Probability Density Function for Non-uniform Air Flow Distribution on the Hydraulic Performance of a Fin-tube Heat Exchanger

Compensation of flow maldistribution in fin-and-tube evaporators for residential air-conditioning

Compensation of flow maldistribution in multi-channel fin-and-tube evaporators for residential air-conditioning is investigated by numerical modeling. The considered sources of maldistribution are distribution of the liquid and vapor phases in the distributor and non-uniform airflow distribution. Fin-and-tube heat exchangers usually have a predefined circuitry, however, the evaporator model is simplified to have straight tubes, in order to perform a generic investigation. The compensation of flow maldistribution is performed by control of the superheat in the individual channels. Furthermore, the effect of combinations of individual maldistribution sources is investigated for different evaporator sizes and outdoor temperatures. It is shown that a decrease in cooling capacity and coefficient of performance by flow maldistribution can be compensated by the control of individual channel superheat. Alternatively, a larger evaporator may be used.

Performance of residential air-conditioning systems with flow maldistribution in fin-and-tube evaporators

Refrigerant and airflow maldistribution in fin-and-tube evaporators for residential air-conditioning was investigated with numerical modeling. Fin-and-tube heat exchangers usually have a pre-defined circuitry. However, the objective in this study was to perform a generic investigation of each individual maldistribution source in an independent manner. Therefore, the evaporator and the condenser were simplified to be straight tubes for the purposes of this study. The numerical model of the R410A system, its verification and an investigation of individual maldistribution sources are presented in this paper. The maldistribution sources of interest were: inlet liquid/vapor phase distribution, feeder tube bending and airflow distribution. The results show that maldistribution reduced the cooling capacity and the coefficient of performance of the system. In particular, different phase distribution and non-uniform airflow distribution reduced the performance significantly. Different feeder tube bendings only caused a minor decrease in performance.

CFD modeling of MPS coal mill with moisture evaporation

Coal pulverizers play an important role in the functioning and performance of a PC-fired boiler. The main functions of a pulverizer are crushing, drying and separating the fine coal particles toward combustion in the furnace. It is a common experience that mill outlet pipes have unequal coal flow in each pipe and contain some coarse particles. Unequal coal flow translates into unequal air-to-fuel ratio in the burner, deviating from the design value and thus increasing unburned carbon in fly ash, NO x and CO. Coarser particles at the mill outlet originate from poor separation and decrease the unit efficiency. In addition, coarser particles reduce burner stability at low load. Air flow distribution at the mill throat, as well as inside the mill, significantly influences the mill performance in terms of separation, drying, coal/air flow uniformity at the mill outlet, wear patterns and mill safety. In the present work, a three-dimensional computational fluid dynamics (CFD) model of the MPS Roll Wheel pulverizer at Alliant Energy's Edgewater Unit 5 has been developed. The Eulerian–Lagrangian simulation approach in conjunction with the coal drying model in Fluent, a commercial CFD software package, has been used to conduct the simulation. Coal drying not only changes the primary air temperature but it also increases the primary air flow rate due to mass transfer from coal. Results of the simulation showed that a non-uniform airflow distribution near the throat contributes significantly to non-uniform air–coal flow at the outlet. It was shown that uniform velocity at the throat improves the air and coal flow distribution at the outlet pipes. A newly developed coal mill model provides a valuable tool that can be used to improve the pulverizer design and optimize unit operation. For example, reject coal rate, which is controlled by the air flow near the mill throat, can be reduced. The model can also be used to further aid in identifying and reducing high temperature or coal-rich areas where mill fires are most likely to start.