Counter-flow Dew Point Evaporative Cooler Research Articles

Theoretical study on the wetting rate in wet channel of dew-point evaporative cooler based on Marangoni effect

Dew-point evaporative cooler is widely concerned as a fresh air precooling system, for its high efficiency, energy saving and environmental protection. A mathematical model for a plate-type counter flow dew-point evaporative cooler is developed in the paper, in which the film shrinkage induced by the Marangoni effect is taken in account. The results show that the model based on Marangoni effect has a higher accuracy compared to the model with wetting rate σ = 100%, in which the Average Relative Deviation from the experiment decreases by 15.8%. The wettability distribution characteristics are investigated. Along the air flow, the transverse contraction distance of film is varied, which mostly depends on the evaporation process. The higher inlet air temperature, the lower inlet air humidity, the higher velocity of inlet air, the larger channel gap, and the higher ratio of working air to inlet air would cause the higher Marangoni number and lower wetting rate. Further, a constant value of wetting rate 96.2% is recommended to simplify the calculation, while the Average Relative Deviation of the simulated results compared to the actual process is only 0.47%.

Performance evaluation of counter flow dew-point evaporative cooler with a three-dimensional numerical model

• A three-dimensional numerical model of a regenerative dew-point evaporative cooler was developed; • The effects of operating conditions were comprehensively analyzed; • The influence of none-even water distribution position was revealed; • The existing of none water area deteriorated the cooler’s effectiveness; • The water distribution at the entrance or outlet of the wet channel affected the cooler’s performance more prominent. Dew-point evaporative cooling is a promising low-emission cooling technique. It applies latent heat from water evaporation with a special humidity manipulation process, which owns a high efficiency and is environmentally friendly. The influences of its operation and design conditions were comprehensively discussed with current one-dimensional or two-dimensional model previously. However, air flow is regarded as constant along the direction of channel width and the previous models assume that the water film is even-distributed in the wet channels, which deteriorates the accuracy of numerical model. In this paper, a three-dimensional model was developed to simulate the air flow and moisture content transport of a regenerative dew-point evaporative cooler. Different from previous models, this study focusses on the effects of non-even water distribution on cooler performance. Results show that the deviation between experimental test and present three-dimensional numerical simulation was less than 3.77% under different inlet conditions, and the three-dimensional model improved the simulation accuracy by 5.46% compared with the conventional two-dimensional model, which demonstrated the superiority of three-dimensional simulation. Based on the proposed three-dimensional numerical model, the influences of inlet conditions, air intake velocity and working air ratio were parametrically analyzed. Last but not least, the effect of water distribution was revealed. The existence of none water distribution area led to poor cooling performance in general, and it had prominent influences on water evaporation when the positions of none water distribution area were near the entrance or outlet of the wet channel. The developed three-dimensional model and the analysis of water distribution effects can benefit the future design of dew-point evaporative coolers.

Parametric analysis of a counter-flow dew point evaporative cooler

Parametric analysis of a counter-flow dew point evaporative cooler

Experimental Investigation on the Cooling Performance of a Counterflow Dew Point Evaporative Cooler

In this study, an innovative indirect evaporative cooler is presented and experimentally analysed. A prototype was assembled for the experimental investigation of the system and tested in the laboratory environment under the conditions of constant air flow rate of 350 m3/h, circulating water temperatures of 15°C and 20°C, inlet air temperatures of 25°C and 30°C and lastly inlet air humidity of 9g/kg and 13g/kg, respectively. Based on the data obtained during the experiments, calculations were performed for the cooling capacity, cooling efficiency, energy efficiency and exergy efficiency of the system for each case. The findings showed that the highest wet bulb efficiency, dew point efficiency and EER of evaporative cooler were found to be 0.91, 0.62, and 0.77, respectively.

Towards a thermodynamically favorable dew point evaporative cooler via optimization

Dew point evaporative cooling is an essential alternative to conventional vapor compression chillers in air conditioning systems to reduce electricity consumption and carbon emission. In this paper, we propose a robust optimization framework of the dew point evaporative cooler towards favorable dew point effectiveness, cooling capacity and Coefficient of Performance (COP). Two optimization algorithms, i.e., multi-to-single-objective and multi-objective optimizations, are developed using genetic algorithm. Concurrently, a 2-D thermodynamic model is established for a counter-flow dew point evaporative cooler and coupled with the optimization algorithms. The model agrees well with the experimental tests on a cooler prototype, and the maximum discrepancy is within ±4.7%. The proposed optimization study is then carried out to investigate the ultimate objective functions and their corresponding decision variables. Key findings that have emerged from this study reveal that the multi-to-single-objective optimization is able to obtain appropriate objective functions according to predefined preference with less complexity and faster response, compared to a multi-objective optimization. The optimal channel length and working ratio are found to be constant in different scenarios, i.e., at 0.50 m and 0.40, respectively, hence they can be eliminated to reduce the number of decision variables.

On the exergy analysis of the counter-flow dew point evaporative cooler

The dew point evaporative cooler has been proposed to replace the mechanical vapor compression chiller in air sensible cooling, for its significantly larger energy efficiency and simpler system layout. Many of the existing studies focused on applying a first-law thermodynamic analysis to the dew point evaporative cooler, however, its performance involving the second-law thermodynamic assessment remains unclear. Therefore, in this paper, an exergy analysis of the counter-flow dew point evaporative cooler is conducted. The exergy performance of the dew point evaporative cooling process is examined by incorporating the first law of thermodynamics for energy and mass balances. A counter-flow dew point evaporative cooler prototype has been designed, fabricated and tested to investigate its cooling performance. A 2-D computational fluid dynamics (CFD) model is then formulated to simulate the flow, temperature and humidity fields of the cooler. The model agrees well with the acquired experimental data with the maximum discrepancy of ±5.6%. The exergy flow, efficiency and efficiency ratio of the cooler are discussed under various simulation conditions. Key findings that emerged from this study reveal that the saturated air state at ambient temperature is the rational dead state to properly describe the physical mechanisms involved in the dew point evaporative cooling process. The exergy efficiency ratio of the dew point evaporative cooler is greater than 1.0, highlighting a remarkable second-law efficiency for air conditioning applications.

A new method for prediction and analysis of heat and mass transfer in the counter-flow dew point evaporative cooler under diverse climatic, operating and geometric conditions

Dew point evaporative cooler, regarded as a zero polluting and energy efficient cooling device, has evolved to be a key technology in air-conditioning systems. The water evaporating process in the cooler is a key performing factor as it leads to the heat sink phenomenon. The cooling effectiveness is dictated by its heat and mass transfer coefficients. The conventional methods (mean temperature difference and integration methods) of obtaining these coefficients have limitations. In this work, a new method to determine these coefficients is proposed. Firstly, a NTU-Le-R model is installed to detect these coefficients. It is based on the outlet data of the dew point evaporative cooler. Next, a two-dimensional computational fluid dynamic model is developed to simulate the evaporative cooling process within the cooler and compute the outlet data for the NTU-Le-R model. Upon validation, results from the computational fluid dynamic model demonstrate close agreement to within ±6.0% with results acquired from experiments. Finally, the effects of the various conditions on the heat and mass transfer coefficients, including climatic, operating and geometric conditions, are judiciously investigated. The new proposed method has the capability to capture the essential boundary conditions to precisely obtain the transfer coefficients. In contrast to existing practices that combine the assumption of the Nusselt number under constant surface heat flux or temperature conditions with the Chilton-Colburn analogy. This new method simplifies computation while providing accurate data to realize optimum design of the dew point evaporative cooler.

The counter-flow dew point evaporative cooler: Analyzing its transient and steady-state behavior

The counter-flow dew point evaporative cooler offers a markedly improved approach to air cooling instead of the conventional vapor compression chiller. Earlier studies have focused on investigating the steady-state cooling effectiveness and energy efficiency of the evaporative cooler. However, there exists limited knowledge of the cooler’s transient characteristics and flow resistance. In addition, existing cooler prototypes mostly employ a water distribution system to spray the water into the wet channel, whereas the effect of water spray remains unclear. Therefore, in this paper, we present a transient and steady-state analysis of the counter-flow dew point evaporative cooler. The channel plate temperature development after water spray was measured and analyzed. A cooler prototype was designed and engineered with a horizontal orientation according to the test results, and a 2-D mathematical model was developed to simulate its performance. The model was able to accurately predict the product air temperature, cooling effectiveness, cooling capacity and COP with a maximum discrepancy of ±5.0%. Key results that emerged from this study revealed that the transient responses of the channel plate and the cooler agreed well with an exponential decay function. The pressure drops for the dry and wet channels spanned 16.0–29.1 Pa and 19.9–52.3 Pa, respectively. The achieved product air temperature ranged from 15.9 to 23.3 °C, with a COP spanning 8.6–27.0.

Similarity analysis and comparative study on the performance of counter-flow dew point evaporative coolers with experimental validation

This paper entails a comparative study on the performance of counter-flow dew point evaporative coolers with two different configurations (type A and type B). Type A refers to the flow configuration where the supply air flows parallel to the water film, while type B refers to the flow configuration where the supply air flows counter to the water film. Both cooler types, when compared with a conventional indirect evaporative cooler, have better potential of lowering the product air temperature below its wet bulb temperature approaching the dew point temperature. A two-dimensional computational fluid dynamics model based on the continuity, momentum, energy and diffusion equations is firstly formulated and then employed to simulate the heat and mass transfer processes. The model, when validated with experimental date, shows a maximum discrepancy of 6.0%. A similarity analysis is then performed to structure the original governing equations of the model into dimensionless forms so as to evolve a fundamental platform that allows key dimensionless parameters to be determined. By regulating these key dimensionless parameters, distributions of the dew point and wet bulb effectiveness and the dimensionless product temperature are plotted via numerical simulation method. Additionally, key simulated data are regressed to obtain the empirical correlation of the dimensionless product air temperature. The key findings that emerged from the present study include: (1) the key dimensionless parameters that are essential to evaluate the performance of the counter-flow dew point evaporative cooler are supply air Reynolds number, water Reynolds number, working air to supply air mass flow rate ratio, water inlet dimensionless temperature, channel length to half width of dry channel ratio and half width of wet channel to half width of dry channel ratio; (2) comparatively, type B configuration has a higher cooling effectiveness and lower product temperature than type A configuration; and (3) the developed dimensionless product air temperature correlation for type B adhered closely to simulated results.

On the fundamental heat and mass transfer analysis of the counter-flow dew point evaporative cooler

The performance of the dew point evaporative cooling (DPEC) is dominated by its convective heat and mass transfer mechanism. Existing mathematical models are mainly developed for the thermodynamic analysis of DPEC under various operating and geometric conditions. The convective heat and mass transfer coefficients are estimated using the Nusselt number and Sherwood number at constant surface conditions. However, as the channel surface is subjected to a naturally-formed boundary condition, the cooler’s actual heat and mass transfer performance remains unclear and has never been investigated. Therefore, we propose an experimental and numerical study, to examine at the fundamental level, the convective heat and mass transfer process of the DPEC. The temperature and humidity distributions of a counter-flow dew point evaporative cooler are measured under different test conditions. The magnitude of the convective heat and mass transfer coefficients are determined using the log mean temperature/humidity difference method. Concurrently, a 2-D mathematical model has been formulated to simulate the heat and mass transfer performance of the cooler. The model agrees well with the acquired experimental data with a maximum discrepancy of ±7.0%. The product air temperature, convective heat and mass transfer coefficients and the Nusselt number and Sherwood number, are further examined under different conditions. Key findings emerged from this study reveal that ReD,r,HL,δH and π are the dominant factors related to the heat and mass transfer performance. The average convective heat and mass transfer coefficients are found to be 26.8–29.9 W/(m2·K) and 0.025–0.027 m/s. The corresponding Nu‾D,d,Nu‾D,w and Sh‾D,w span 8.67–9.95, 8.68–9.21 and 8.17–8.67, respectively, under varying dimensionless numbers/groups.

The heat and mass transfer process of the counter-flow dew point evaporative cooler

The dew point evaporative cooling technology is a potential substitute for the conventional vapor compression chillers in the air conditioning systems. Most of the existing studies focus on the thermodynamic analysis of the dew point evaporative cooler under various conditions. As the dominant physical mechanism, the in-depth heat and mass transfer phenomena have yet been studied. In this paper, we propose a detailed investigation on the heat and mass transfer process of the counter-flow dew point evaporative cooler. An experimental setup was established to measure the temperature or humidity distributions of the supply air, working air and water film. The heat and mass transfer coefficients were calculated via the temperature and humidity distributions. Concurrently, a 2-D mathematical model was developed to predict the experimental results of the cooler based on the momentum, continuity, energy and species balance equations. Key results that emerged from this study include: (1) the governing dimensionless numbers/groups in the cooling process are determined; (2) the Nusselt number and Sherwood number, which reveal the intensity of the convective heat and mass transfer, are studied with reference to those dimensionless groups.

Thermodynamic analysis of a hybrid membrane liquid desiccant dehumidification and dew point evaporative cooling system

The desiccant-enhanced dew point evaporative cooling system is proposed as a potential alternative for air conditioning. It enables independent control of the air humidity and temperature so that latent and sensible heat loads are judiciously decoupled. In this paper, we carried out a thermodynamic analysis on a hybrid membrane liquid desiccant dehumidification and dew point evaporative cooling system. A cross-flow membrane liquid desiccant dehumidifier and a counter-flow dew point evaporative cooler were developed and investigated. Experiments were conducted to demonstrate the ability of the dehumidifier and cooler to achieve sufficient moisture removal and temperature reduction. The supply air could be regulated to the suggested thermal comfort zone of 20.0–28.0 °C temperature with humidity ratio of below 12.0 g/kg. A detailed mathematical model was formulated by considering the momentum, continuity, energy and species equations. The model was able to predict well the experimental data with a maximum discrepancy of ±5.0%. Key findings that emerged from this study include: (1) the proposed system was able to deliver the product air at 18.3 °C temperature and 10.9 g/kg humidity under nominal conditions; (2) the channel heights of the dehumidifier and cooler should be maintained at 2.0 and 3.0 mm, with their channel lengths at 0.2 and 0.6 m, respectively, to achieve thermal comfort conditions; and (3) a solution flow ratio of 2.7 with the membrane diffusion resistance not larger than 20 s/m achieved sufficient humidity reduction of more than 9.0 g/kg.

Investigation of dew point evaporative cooling with vacuum membrane dehumidification

The vacuum membrane dehumidification and dew point evaporative cooling are potential technologies to handle the latent and sensible heat of the air, respectively. This paper proposes a hybrid system of vacuum membrane dehumidifier and dew point evaporative cooler for air conditioning. A horizontal counter-flow dew point evaporative cooler was designed and developed. The transient and steady-state characteristics of the cooler were judiciously examined. The cooling potential of the cooler itself and the combined system with a membrane dehumidifier were then investigated and compared. Key findings that emerged from this study include: (1) the dew point evaporative cooler alone delivers the product air temperature at 24.8 °C under the hot and humid condition, with the wet bulb effectiveness of 1.05; (2) the hybrid cooling system is able to reduce the ambient humidity to 8.2 g/kg, and the ambient temperature is cooled to below 18.5 °C.

Multivariate scaling and dimensional analysis of the counter-flow dew point evaporative cooler

The theoretical study of the counter-flow dew point evaporative cooler is currently limited to the influence of different operating conditions and geometric parameters. Therefore, this paper presents an in-depth investigation on the governing factors of the dew point evaporative cooling process. A 2-D mathematical model has been developed based on the momentum, continuity, energy and species balance equations. The model was able to achieve good agreement with a maximum discrepancy of ±8.0% in predicting the transient and steady-state performance of the counter-flow dew point evaporative cooler. A new dimensionless model was produced from the general model via scaling analysis. The relevant dimensionless groups controlling the operating conditions, geometric parameters and supply air conditions were derived. We further discussed the relative importance of the physical mechanisms involved in the heat and mass transfer process. In addition, a dimensional analysis was carried out to investigate the quantitative expressions of the dimensionless time constant and product air temperature. The key findings that emerged from this study are: (1) the dominant dimensionless groups for the evaporative cooling process are Re,r,HL,δH,T0∗ and π; and (2) the correlations for the dimensionless time constant and product air temperature have been developed and validated with data to demonstrate good accuracy.

Fundamental formulation of a modified LMTD method to study indirect evaporative heat exchangers

An analytical model for indirect evaporative heat exchangers has been developed via a modified log mean temperature difference (LMTD) method designed for sensible heat exchangers. The original LMTD method is judiciously modified to extend its computing method to indirect evaporative cooling systems where latent heat transfer is involved. The analytical model is validated by comparing its prediction of thermal performance with experimental data of a counter-flow dew-point evaporative cooler and a cross-flow indirect evaporative cooler. Predictability of the modified LMTD method has a maximum discrepancy of ±8% when compared to experimental data. The model has been demonstrated to be a practical method to provide an accurate result with a short computational time. Several case studies are structured to illustrate that the modified LMTD method is suitable for designing and analyzing indirect evaporative heat exchangers.