Transient Behavior of Three-Fluid Cross-Flow Heat Exchanger Under the Influence of Temperature Nonuniformity

2018 ◽  
Vol 10 (6) ◽  
Author(s):  
Harpreet Kaur Aasi ◽  
Manish Mishra
Keyword(s):  
Heat Exchanger ◽  
Thermal Stresses ◽  
Cross Flow ◽  
Transient State ◽  
Transient Behavior ◽  
Inlet Temperature ◽  
State Behavior ◽  
Temperature Nonuniformity ◽  
Fluid Properties ◽  
Flow Heat

Abstract A typical three-fluid cross-flow heat exchanger with nonuniform inlet temperature in the central (hot) fluid is considered for the present analysis. Steady and transient state behavior of the heat exchanger is observed for four different temperature nonuniformity models along with step excitation in inlet temperature of the central fluid. Longitudinal heat conduction in the separating walls and the effect of fluid back-mixing along with axial dispersion effect are considered within the fluids with constant thermophysical fluid properties. The solution of governing equations has been obtained using implicit finite difference scheme. Temperature distribution over the separating walls has been depicted providing a clear view of the thermal stresses generated in separating walls. The performance for all the four cross-flow arrangements has been analyzed by comparing that with and without nonuniform conditions. It is found that the nonuniformity in inlet temperature has an adverse effect on the performance of heat exchanger.

Steady and Transient Behavior of Cross-Flow Three-Fluid Heat Exchanger With Temperature Nonuniformity in All the Fluids—A Detailed Investigation

2019 ◽  
Vol 141 (11) ◽  
Author(s):  
Harpreet Kaur Aasi ◽  
Manish Mishra
Keyword(s):  
Heat Exchanger ◽  
Cross Flow ◽  
Transient Behavior ◽  
Operating Conditions ◽  
Inlet Temperature ◽  
Transient Conditions ◽  
Finite Difference Technique ◽  
Temperature Nonuniformity ◽  
Exit Temperature ◽  
Implicit Finite Difference

Abstract Cross-flow three-fluid plate-fin heat exchanger is analyzed under both steady-state and transient conditions with a nonuniform inlet temperature of all the three fluids. The influence of the longitudinal heat conduction and axial dispersion in the separating sheets and three fluids, respectively, is also considered. Five different combinations (modes) of temperature nonuniformity in the three fluids have been considered and compared for the performance. An important phenomenon of temperature cross between/among the fluids has been observed and presented for certain modes of temperature nonuniformity and operating conditions. The effect in the performance has been presented on the basis of mean exit temperature and deterioration factor. Implicit finite difference technique has been used for the numerical solution. The heat exchanger's performance is found to be dependent on the mode of temperature nonuniformity, number of transfer units, and the operating parameters.

Consideration of Uncertainties in Compact Cross-Flow Heat Exchanger Design for Gas Turbine Engine Application

2013 ◽  
Author(s):  
Randall D. Manteufel ◽  
Daniel G. Vecera
Keyword(s):  
Fluid Flow ◽  
Heat Exchanger ◽  
Gas Turbine ◽  
Flow Rate ◽  
Cross Flow ◽  
Inlet Temperature ◽  
Fluid Flow Rate ◽  
Heat Exchanger Design ◽  
Cold Fluid ◽  
Flow Heat

Recent experimental work characterized the performance of a unique cross-flow heat exchanger design for application of cooling compressor bleed air using liquid jet fuel before it is consumed in the gas turbine combustor. The proposed design has micro-channels for liquid fuel and cools air flowing in passages created using rows of intermittent fins. The design appears well suited for aircraft applications because it is compact and light-weight. A theoretical model is reported to be in good agreement with experimental measurements using air and water, thus providing a design tool to evaluate variations in the heat exchanger dimensions. This paper presents an evaluation of the heat exchanger performance with consideration of uncertainties in both model parameters and predicted results. The evaluation of the design is proposed to be reproduced by students in a thermal-fluids design class. The heat exchanger performance is reevaluated using the effectiveness–NTU approach and shown to be consistent with the method reported in the original papers. Results show that the effectiveness is low and in the range of 20 to 30% as well as the NTU which ranges from 0.25 to 0.50 when the heat capacity ratio is near unity. The thermal resistance is dominated by the hot gas convective resistance. The uncertainties attributed to fluid properties, physical dimensions, gas pressure, and cold fluid flow rate are less significant when compared to uncertainties associated with hot fluid flow rate, hot fluid inlet temperature, cold fluid inlet temperature, and convective correlation for gas over a finned surface. The model shows which heat transfer mechanisms are most important in the performance of the heat exchanger.

Review and Analysis of Cross Flow Heat Exchanger Transient Modeling for Flow Rate and Temperature Variations

2015 ◽  
Vol 7 (4) ◽  
Author(s):  
Tianyi Gao ◽  
James Geer ◽  
Bahgat Sammakia
Keyword(s):  
Heat Exchanger ◽  
Mass Flow Rate ◽  
Transient Response ◽  
Mass Flow ◽  
Flow Rate ◽  
Heat Exchangers ◽  
Cross Flow ◽  
Inlet Temperature ◽  
Flow Rate Change ◽  
Flow Heat

Heat exchangers are important facilities that are widely used in heating, ventilating, and air conditioning (HVAC) systems. For example, heat exchangers are the primary units used in the design of the heat transfer loops of cooling systems for data centers. The performance of a heat exchanger strongly influences the thermal performance of the entire cooling system. The prediction of transient phenomenon of heat exchangers is of increasing interest in many application areas. In this work, a dynamic thermal model for a cross flow heat exchanger is solved numerically in order to predict the transient response under step changes in the fluid mass flow rate and the fluid inlet temperature. Transient responses of both the primary and secondary fluid outlet temperatures are characterized under different scenarios, including fluid mass flow rate change and a combination of changes in the fluid inlet temperature and the mass flow rate. In the ε-NTU (number of transfer units) method, the minimum capacity, denoted by Cmin, is the smaller of Ch and Cc. Due to a mass flow rate change, Cmin may vary from one fluid to another fluid. The numerical procedure and transient response regarding the case of varying Cmin are investigated in detail in this study. A review and comparison of several journal articles related to the similar topic are performed. Several sets of data available in the literatures which are in error are studied and analyzed in detail.

Numerical and Experimental Characterization of the Transient Effectiveness of a Water to Air Heat Exchanger for Data Center Cooling Systems

2015 ◽  
Author(s):  
Tianyi Gao ◽  
Marcelo del Valle ◽  
Alfonso Ortega ◽  
Bahgat G. Sammakia
Keyword(s):  
Heat Exchanger ◽  
Mass Flow ◽  
Data Center ◽  
Heat Exchangers ◽  
Cooling System ◽  
Cross Flow ◽  
Inlet Temperature ◽  
Experimental Measurements ◽  
Cooling Systems ◽  
Flow Heat

The cross flow heat exchanger is at the heart of most cooling systems for data centers. Air/Water or air/refrigerant heat exchangers are the principal component in Central Room Air Conditioning (CRAC) units that condition data room air that is delivered through an underfloor plenum. Liquid/air heat exchangers are also increasingly deployed in close-coupled cooling systems such as rear door heat exchangers, in-row coolers, and overhead coolers. In all cases, the performance of liquid/air heat exchangers in both steady state and transient scenarios are of principal concern. Transient scenarios occur either by the accidental failure of the cooling system or by intentional dynamic control of the cooling system. In either scenario, transient boundary conditions involve time-dependent air or liquid inlet temperatures and mass flow rates that may be coupled in any number of potential combinations. Understanding and characterizing the performance of the heat exchanger in these transient scenarios is of paramount importance for designing better thermal solutions and improving the operational efficiency of existing cooling systems. In this paper, the transient performance of water to air cross flow heat exchangers is studied using numerical modeling and experimental measurements. Experimental measurements in 12 in. × 12 in. heat exchanger cores were performed, in which the liquid (water) mass flow rate or inlet temperature are varied in time following controlled functional forms (step jump, ramp). The experimental data were used to validate a transient numerical model developed with traditional assumptions of space averaging of heat transfer coefficients, and volume averaging of thermal capacitances. The complete numerical model was combined with the transient effectiveness methodology in which the traditional heat exchanger effectiveness approach is extended into a transient domain, and is then used to model the heat exchanger transient response. Different transient scenarios were parametrically studied to develop an understanding of the impact of critical variables such as, the fluid inlet temperature variation and the fluid mass flow rate variation, and a more comprehensive understanding of the characteristics of the transient effectiveness. Agreement between the novel transient effectiveness modeling approach and the experimental measurements enable use of the models as verified predictive design tools. Several studies are designed based on the practical problems related to data center thermal environments and the results are analyzed.

Heat and Mass Transfer to Air in a Cross Flow Heat Exchanger with Surface Deluge Cooling

2016 ◽  
Vol 24 (01) ◽  
pp. 1650002 ◽  
Author(s):  
Andrea Diani ◽  
Luisa Rossetto ◽  
Roberto Dall’Olio ◽  
Daniele De Zen ◽  
Filippo Masetto
Keyword(s):  
Heat Exchanger ◽  
Heat Flow ◽  
Flow Rate ◽  
Data Center ◽  
Heat Dissipation ◽  
Cross Flow ◽  
Critical Conditions ◽  
Heat Flow Rate ◽  
External Temperature ◽  
Flow Heat

Cross flow heat exchangers, when applied to cool data center rooms, use external air (process air) to cool the air stream coming from the data center room (primary air). However, an air–air heat exchanger is not enough to cope with extreme high heat loads in critical conditions (high external temperature). Therefore, water can be sprayed in the process air to increase the heat dissipation capability (wet mode). Water evaporates, and the heat flow rate is transferred to the process air as sensible and latent heat. This paper proposes an analytical approach to predict the behavior of a cross flow heat exchanger in wet mode. The theoretical results are then compared to experimental tests carried out on a real machine in wet mode conditions. Comparisons are given in terms of calculated versus experimental heat flow rate and evaporated water mass flow rate, showing a good match between theoretical and experimental values.

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