Air Side Nusselt Number Research Articles

Transient experimental study of a latent heat thermal energy storage in a heat exchanger for effective thermal management

The rising demands for energy use and the increase in environmental and economic concerns necessitate a reduction in energy consumption of thermal systems. A phase change material (PCM) is a viable solution to enhance thermal system performance through its latent heat thermal energy storage capability. This study experimentally investigates an innovative PCM-air-liquid meso heat exchanger to extend the thermal comfort in a vehicle during short periods of engine shutdown. Extended surfaces (fins) are placed inside the PCM medium and integrated throughout the whole heat exchanger to enable and improve the transfer of thermal energy between the PCM and the air. It is found from the experiments that the PCM extends the airside cooling time by more than 4 min when the main coolant is shutdown. A comparison of heat transfer when using PCM to no PCM case revealed a 120 kJ released from the PCM during the discharging process. Furthermore, doubling the air mass flow rate resulted in a 29 % decrease in the cumulative energy transferred to the air. The presented system offers an innovative solution for the start-stop function implemented in many hybrid and electric vehicles. In addition, a transient response of the crossflow heat exchanger is studied due to airside mass velocity variations. A generalized empirical correlation to predict the airside Nusselt number as a function of air mass velocity ratio is derived.

Cooling Modelling of an Electrically Heated Ceramic Heat Accumulator

This paper presents a simple novel mathematical model of a heat accumulator with an arranged packing in the form of ceramic cylinders. The accumulator analysed in the paper can be heated with inexpensive electricity overnight or excess electricity from wind farms. It can be used as a heat source in a hydronic heating system or for domestic hot water. The differential equations describing the transient temperature of the accumulator packing and flowing air were solved using the explicit Euler and Crank–Nicolson methods. The accuracy of both methods was assessed using exact analytical solutions and the superposition method for a uniform initial temperature and accounted for time changes in inlet air temperature. A numerical simulation of the accumulator cooled by flowing air was carried out. The correlation for the air-side Nusselt number was determined using the method of least squares based on experimental data. The calculated exit air temperature was compared with the measured data. The accumulator can operate as a heat source with dynamic discharge. The developed mathematical model of the accumulator can be used in a system to adjust the fan rotational speed so that the air temperature in the room is equal to the preset temperature.

Comparison of Individual Air-Side Row-By-Row Heat Transfer Coefficient Correlations on Four-Row Finned-Tube Heat Exchangers

Each row of the heat exchanger (HEX) has different hydraulic-thermal characteristics. When the air velocity upstream of the HEX is lower than approximately 3 m/s, the exchanged heat flow rates at the first rows of tubes are higher than the average value for the entire HEX. The article presents the determination of the individual correlations for the air-side Nusselt numbers on each row of tubes for a 4-row finned HEX with continuous flat fins and round tubes in a staggered tube layout. The method was built based on computational fluid dynamics modeling. Mass average temperatures for each row were simulated for over a dozen different airflow velocities, from 2.5 m/s to 10 m/s. The obtained individual correlations for the air-side Nusselt number will enable selecting the optimum number of tube rows for a given heat output of the HEX. The investment costs of the HEX can be reduced by decreasing the tube row number. Moreover, the operating costs of the HEX can also be lowered, as the air pressure losses on the HEX will be lower, which in turn enables the reduction of the air fan power.

Numerical Modelling of Rapid Decrease in Water Flow Rate in a Plate-and-Tube Heat Exchanger

In this article, a mathematical simulation of a plate-fin and tube heat exchanger (PFTHE) is presented. Water flows in the tubes and air flows perpendicular to the tube axis. The general numerical model was developed to perform the transient operation of the heat exchanger. The Reynolds number on the waterside ranged from 4,000 to 12,000. The transient response of the PFTHE was modeled for a rapid decrease in the water volume flow rate in time. The heat transfer correlation for the air side was established using the empirical data. Unknown parameters present in the power-type relationship for the airside Nusselt number were estimated using the least-squares method. The waterside heat transfer coefficients in the transitional and turbulent flow ranges were calculated using the semi-empirical heat transfer correlation. The results of the mathematical simulations of the investigated PFTHE were compared with the measurement data. The agreement between computation and measurement results, i.e., between the calculated and measured air and water temperatures at the outlets of the PFTHE, was very satisfactory.

Air-Side Nusselt Numbers and Friction Factor’s Individual Correlations of Finned Heat Exchangers

Currently, when designing finned heat exchangers (FHE), the average value of the entire heat transfer coefficient (HTC) is considered. However, each row of the heat exchanger (HEX) has different hydraulic-thermal characteristics. The novelty of this research is to present the differentiation of the individual air-side Nusselt number and Darcy-Weisbach friction factor correlations in each row of FHE using CFD modelling. FHE has four-rows, circular tubes, and continuous fins with a staggered tube arrangement. Relationships for the Nusselt number and D-W friction factor derived for the entire exchanger based on CFD modelling were compared with those available in the literature, determined using experimental data. The maximum relative differences between the Nusselt number for a four-row FHE determined experimentally and by CFD modelling are in the range from 22% for a Reynolds number based on a tube outside diameter of 1000 to 30% for a Reynolds number of 13,000. The maximum relative differences between the D-W friction factor for a four-row FHE determined experimentally and by CFD modelling are in the range of 50% for a Reynolds number based on a tube outer diameter of 1000 to 10% for a Reynolds number of 13,000. The CFD modelling performed shows that in the range of Reynolds numbers based on hydraulic diameters from 150 to 1400, the Nusselt number for the first row in a four-row FHE is about 22% to 15% higher than the average Nusselt number for the entire exchanger. In the range of Reynolds number changes based on hydraulic diameter from 2800 to 6000, the Nusselt numbers on the first and second rows of tubes are close to each other. Correlations of Nusselt numbers and D-W friction factors derived for individual tube rows can be used in the design of plate-fin and tube heat exchangers used in equipment such as air-source heat pumps, automotive radiators, air-conditioning systems, and in air hot-liquid coolers. In particular, the correlations can be used to select the optimum number of tube rows in the exchanger.

Structural Optimization of Self-Supporting Rectangular Converging-Diverging Tube Heat Exchanger

A three-dimensional numerical investigation of turbulent heat transfer and fluid flow characteristics of the new heat exchanger and self-support of a rectangular converging-diverging (SS-RCD) tube bundle heat exchanger with different inserts was performed. The values of the Reynolds number varied from 27,900 to 41,900. The baseline case (without an insert) was compared with two enhanced configurations: one circular hole in the baffle plate (one-circle case) and a rectangular hole in the baffle plate (one-rectangle case). Compared with the baseline case, the airside Nusselt number (Nu) of the enhanced cases improved by 39.6~48.0% and 36.2~40.2% and had an associated friction factor (f) penalty increase of 53.9–66.7% and 60.7–77.8%, respectively. The baseline case was compared with three enhanced configurations: one-circle case, two-circle case, and three-circle case baffle plate. Compared with the baseline case, Nu of the enhanced cases improved by 39.6–48.0%, 36.2–45.4%, and 35.0–44.2%, with f penalty increases of 53.9–66.7%, 44.9–60.0%, and 43.8–60.0%, respectively. The overall performance was conducted by heat transfer enhancement factor (η). It was found that the one circle case obtained the best overall performance. The numerical results were analyzed from the viewpoint of the field synergy principle. It was found that the reduction in the average intersection angle between the velocity vector and the temperature gradient (θ) was one of the essential factors influencing heat transfer performance.

Multiobjective geometry optimization of microchannel heat exchanger using real-coded genetic algorithm

• A weight-based, real-coded genetic algorithm optimization was performed. • Air-side Nusselt number, friction factor, and fin efficiency were modeled. • The fin pitch imposed the most significant effect on the volume and the fan power. • The microchannel heat exchanger became more compact at higher target heat duties. • The optimization approach provides optimal solutions at given design dimensions. In this paper, a multiobjective optimization of the structure of a flat-tubed microchannel heat exchanger is performed to reduce its volume and fan power at a specified capacity. Design variables include tube height, tube width, tube length, fin height, and fin pitch. A weight-based, real-coded genetic algorithm is implemented to optimize the design variables within their specified range of dimensions. To further improve the numerical simulations of the microchannel heat exchanger performance, correlations for the air-side Nusselt number, friction factor, and fin efficiency are developed and validated. In the optimization, the Pareto optimal fronts are obtained by varying weights of the two conflicting objectives. A reference microchannel heat exchanger operating at different capacities is optimized. Results show that the volume and fan power of the reference microchannel heat exchanger can be reduced by up to 45% and 51% respectively, depending on the weighting factor selected. The optimization approach of this study provides the optimal solutions at the given domain of geometric parameter dimensions.

Transient experimental investigation of airside heat transfer in a crossflow heat exchanger

• Experimental analysis is performed to study the dynamic response of a heat exchanger. • Residence time is found to have a significant effect on the response time. • Transient air heat transfer coefficient is obtained by energy balance on the wall. • An empirical correlation of airside Nusselt number is developed. • The response time of both fluids was not instantaneous to the step imposed. The increase in energy consumption and the demand for effective thermal management systems necessitate the search for enhanced heat exchangers with high thermal performance, lower weight, and compact size. Crossflow heat exchangers are a key component in the thermal management system of many industries such as HVAC, automotive, etc. Transient analysis helps predict the behavior of heat exchangers when they experience variations in their operational conditions in terms of flow rate or temperature. This work experimentally studies the dynamic response of a cross flow heat exchanger to overcome the data scarcity in transient airside and presents the results for various cases of step changes in air mass flow rate. The effect of the step change on heat transfer is investigated and presented by airside Nu, fluids dimensionless temperature, airside Re, heat transfer rates, and Colburn j factor. Results show that the two fluids do not respond instantaneously to the step imposed. The magnitude of the step change has a dissimilar effect on the response time of each fluid. The response time increases with the increase in the step change. A generalized empirical correlation is obtained to predict the airside Nu. This study advances the control of heat exchangers by exploring their transient response due to step changes in one of the fluids inlet condition.

Thermal Calculations of Four-Row Plate-Fin and Tube Heat Exchanger Taking into Account Different Air-Side Correlations on Individual Rows of Tubes for Low Reynold Numbers

Currently, when designing plate-fin and tube heat exchangers, only the average value of the heat transfer coefficient (HTC) is considered. However, each row of the heat exchanger (HEX) has different hydraulic–thermal characteristics. When the air velocity upstream of the HEX is lower than approximately 3 m/s, the exchanged heat flow rates at the first rows of tubes are higher than the average value for the entire HEX. The heat flow rate transferred in the first rows of tubes can reach up to 65% of the heat output of the entire exchanger. This article presents the method of determination of the individual correlations for the air-side Nusselt numbers on each row of tubes for a four-row finned HEX with continuous flat fins and round tubes in a staggered tube layout. The method was built based on CFD modelling using the numerical model of the designed HEX. Mass average temperatures for each row were simulated for over a dozen different airflow velocities from 0.3 m/s to 2.5 m/s. The correlations for the air-side Nusselt number on individual rows of tubes were determined using the least-squares method with a 95% confidence interval. The obtained correlations for the air-side Nusselt number on individual rows of tubes will enable the selection of the optimum number of tube rows for a given heat output of the HEX. The investment costs of the HEX can be reduced by decreasing the tube row number. Moreover, the operating costs of the HEX can also be lowered, as the air pressure losses on the HEX will be lower, which in turn enables the reduction in the air fan power.

Thermal calculations of plate–fin–and-tube heat exchangers with different heat transfer coefficients on each tube row

In finned tube heat exchangers, the first rows of tubes are most effective when the air velocity in front of the exchanger is less than about 3.5 m/s. The heat transfer coefficient (HTC) decreases with each subsequent row and stabilizes only from the fifth row onwards. The paper examines a two-pass double-row plate-fin and tube heat exchanger (PFTHE) made of circular or oval pipes. A method for determining the air side Nusselt number on individual pipe row was developed, using the results of CFD (Computational Fluid Dynamics) modelling of the heat exchanger. Also, a heat transfer correlation was found for the mean HTC for the whole PFTHE using CFD modelling. The correlations based on the results of CFD modelling match very well the empirical correlations based on experimental tests of two car radiators. The analysis shows that the first rows of PFTHE are the most effective in terms of thermal effectiveness. By building a heat exchanger with two rows of pipes, it is possible to reduce investment expenditures at the same thermal output significantly. Heating systems with PFTHE air heaters with fewer rows of pipes are characterized by lower fuel consumption due to their higher efficiency. The investment costs for automotive radiators can also be reduced. The introduction of PFTHE heat exchangers with fewer pipe rows has a positive impact on the environment. Both material expenditures on the construction of PFTHE, energy consumption by the air fan, and fuel consumption for heating purposes are reduced.

Transient response of a plate-fin-and-tube heat exchanger considering different heat transfer coefficients in individual tube rows

Experimental studies of multi-row plate-fin heat exchangers show that the highest average heat transfer coefficient on the air side occurs in the first row of tubes. In the next rows of tubes up to about the sixth row the heat transfer coefficient decreases.It is necessary to find the relationships for the air-side Nusselt number on each tube row to design a plate-fin and tube heat exchanger (PFTHE) with the optimum number of tube rows. The air-side Nusselt number correlations can be determined experimentally or by CFD modeling (Computational Fluid Dynamics). The paper presents a new method of modeling the transient operation of PFTHE, considering that the Nusselt numbers on the air side of individual tube rows are calculated from different empirical relationships. CFD modeling of the PFTHE was used to establish the heat transfer coefficients on individual tube rows. A transient model of a double-row, two-pass PFTHE was developed, considering different heat transfer coefficients in the first and second row of tubes. The influence of the adoption of different coefficients on the heat flow rate exchanged between water and air in the first and second row of tubes in two passes of the heat exchanger was shown.

Transient behavior of a plate-fin-and-tube heat exchanger taking into account different heat transfer coefficients on the individual tube rows

Plate-fin and tube heat exchangers (PFTHE) are made of round, elliptical, oval or flat tubes to which continuous fins ( lamellas) are attached. Liquid flows inside the tubes and gas flows outside the tubes perpendicularly to their axes and parallel to the surface of continuous fins. Experimental studies of multi-row plate-fin and tube heat exchangers show that the highest average heat transfer coefficient on the air side occurs in the first row of tubes when the air velocity in front of the exchanger is less than approximately 3.5 m/s when a Reynolds number based on an equivalent hydraulic diameter equal to the distance between tube rows in the direction of air flow is less than 10,000. In the subsequent rows of tubes up to about the fourth row the heat transfer coefficient decreases. In the fifth and further rows, it can, that the heat transfer coefficient is equal in each tube row. It is necessary to find the relationships for the air-side Nusselt number on each tube row to design a PFTHE with the appropriate number of tube rows. The air-side Nusselt number correlations can be determined experimentally or by CFD modeling (Computational and Fluid Dynamics). The paper presents a new mathematical model of the transient operation of PFTHE, considering that the Nusselt numbers on the air side of individual tube rows are different. The heat transfer coefficient on an analyzed tube row was determined from the equality condition of mass- average air temperature differences on a given tube row determined using the analytical formula and CFD modeling. The results of numerical modeling were compared with the results of the experiments.

Transient behavior of a plate-fin-and-tube heat exchanger taking into account different heat transfer coefficients on the individual tube rows

Experimental studies of multi-row plate-fin heat exchangers show that the highest average heat transfer coefficient on the air side occurs in the first row of tubes when the air velocity in front ofthe exchanger is less thanapproximately 3.5 m/s. In the subsequent rows of tubes up to about the fourth row the heat transfer coefficient decreases. In the fifth and further rows, it can be assumed that the heat transfer coefficient is equal in each tube row. It is necessary to find the relationships fortheair–side Nusselt number on each tube row to design a plate–fin and tube heat exchanger(PFTHE) with the appropriate number of tube rows. The air–side Nusselt number correlations canbe determined experimentally or by CFD modeling (Computational and Fluid Dynamics). The paper presents a newmathematical model of the transient operation of PFTHE, considering that the Nusselt numbers on the air side of individual tube rows are different. The heat transfer coefficient on an analyzed tube row was determined from the equality condition of mass– average air temperature differences on agiven tube row determined using the analytical formula and CFD modeling. The results of numerical modelingwere compared with the results of the experiments.

Numerical modeling transient response of tubular cross flow heat exchanger

PurposeThe paper aims to present the mathematical modeling of plate fin and tube heat exchanger at small Reynolds numbers on the water side. The Reynolds number of the water flowing inside the tubes was varied in the range from 4,000 to 12,000.Design/methodology/approachA detailed analysis of transient response was modeled for the following changes in the operating parameters of the heat exchanger: a reduction in the water volume flow, an increase in the water volume flow and an increase in the water volume flow with a simultaneous reduction in the air flow velocity.FindingsThe results of the numerical simulation of a heat exchanger by using experimentally determined water-side heat transfer correlation and theoretical correlation derived for the transition tube flow agree very well. The relationship to calculate the air-side Nusselt number was determined experimentally. The correlation for the air-side Nusselt number was the same for the theoretical and experimental water side correlation.Research limitations/implicationsThe correlation for the air-side Nusselt number as a function of the Reynolds and Prandtl numbers is based on the experimental data and was determined using the least squares method.Originality/valueThe form of the relationship that was used to approximate experimentally determined water-side Nusselt numbers is identical to the theoretically derived formula for the transition range. The experiments show that the relationship for the water-side Nusselt number in transition and turbulent flow regime that was obtained using theoretical analysis gives quite satisfactory results.

Experimental study on the effects of fin pitches and tube diameters on the heat transfer and fluid flow characteristics of a fin punched with curved delta-winglet vortex generators

The effects of fin pitches and tube diameters on the air side heat transfer performance of a tube bank fin heat exchanger using curved delta-winglet vortex generators (CDVGs) were experimentally investigated, and the counterpart plain fin was selected as a reference for optimizations of fin geometries. Totally 18 samples with three different fin pitches and three different tube diameters were studied for the fin with/without CDVGs. Based on the experimental data and their corresponding fitting formula, the effects of the fin pitches and tube diameters on heat transfer performance of the fin with or without CDVGs were discussed, and the correlations of air side Nusselt number and friction factor with Reynolds numbers, fin pitches and tube diameters were obtained. The results indicate that the fin patterns with CDVGs could effectively enhance heat transfer performance compared with the counterpart plain fin under identical pump power constraint, and their heat transfer enhancement performances are dependent on the flow condition and the fin’s geometry parameters. In the range of studied geometry parameters and Reynolds numbers, the best performance could be achieved with the combination of the largest fin pitch and the smallest tube diameter, and the optimum parameters are less influenced by the Reynolds number.

Mathematical modeling of unsteady response of plate and fin heat exchanger to sudden change in liquid flow rate

A mathematical simulation of plate fin and tube heat exchanger will be presented. The transient operation of the heat exchanger was carried out using general numerical model developed in [1]. Reynolds number of the water flowing inside the tubes varied in the range from 4000 to 12000. A detailed analysis of transient response was modeled for growth in the water volume flow in time.At first, heat transfer correlations for air and water were determined based on the experimental data. Unknown parameters appearing in the relationships for the Nusselt numbers on the air and water-sides were estimated using the least squares method. The power-type form of the relationship for the air-side Nusselt number was used while two correlations of different form were chosen for the water-side Nusselt number. The first is the exponential correlation, and the second form is similar to the relationships of Petukhov-Kirillov and Gnielinski. These correlations were used in the mathematical model of the heat exchanger for the simulation of its transient operation.The results of the numerical simulations of a heat exchanger using experimentally determined air and water-side heat transfer formulas for calculation of heat transfer coefficient were compared with the experimental data. Very good agreement of computation results (i.e. air and water temperature at the outlet of the heat exchanger) with the experimental data was obtained.

Fabrication method and thermal-frictional behavior of a tube-in-tube helically coiled heat exchanger which contains turbulator

No type of turbulator was used for tube-in-tube helically coiled (TTHC) heat exchangers in previous academic investigations or industrial applications. It seems that, the first challenge is related to the manufacture method of TTHC heat exchanger which contains turbulator. Hence, the first step of this paper presents a technique which was used to fabricate a TTHC heat exchanger with and without turbulator for this study. And then, the effects of aforesaid turbulator on thermal and frictional characteristics are experimentally investigated. Hot water was employed for outer tube of heat exchanger for all experiments in this research; and cold air flow or cold water flow were utilized as working fluids of inner tube. Memorable results were obtained in this study. Findings showed that, the use of turbulator only for outer tube (hot water side) increases the air side Nusselt number (inner tube) around 8–32%. The employment of turbulator only for inner tube (air side) enhances the Nusselt number around 52–81%. Utilization of turbulator for air side marginalizes the effect of water side turbulator and it has no longer tangible effect.

Using Evaporative Cooling Methods for Improving Performance of an Air-cooled Condenser

In areas with very hot weather conditions (50 to 60℃), the temperature and pressure of the air-conditioning condenser are increased considerably. This causes a decrease in the cooling capacity of the cycle and also causes an increase in the power consumption due to increased pressure ratio. In this work, an experimental and theoretical investigation has been done to improve the evaporator outlet fluid temperature through enhancing condenser performance. For this purpose, several modifications on the refrigeration system have been developed and tested to solve this hot weather problem. The air-side modifications include adding Spray Water above condenser (SW), wet Pad before condenser (Pad), and water Vapor Nozzle in the condenser air flow (VN). The refrigerant-side modification includes adding a pair of Heat Exchangers (HE) for exchanging heat between condenser exit and evaporator exit by using water-antifreeze mixture as a working fluid. A Water-Refrigerant(W-R) evaporator has been designed, manufactured, and compared with original Air-Refrigerant(A-R) evaporator performance. All air and refrigerant-side modifications have been investigated using both types of evaporators. The results indicate that the (SW) modification for enhancing condenser performance is the best method for COP improvement. The COP of (SW) system is found to increase at rate of (44.5 %) and (102.1%) as compared to system without modifications for (A-R) and (W-R) evaporators respectively. The outlet cooling temperature from evaporator has been found to reduce by about (30.3%) for (A-R) evaporator and (23.6%) for (W-R) evaporator. However, (HE+Pad) modification has been found as the best method for improving air side Nusselt number of condenser with an increase of about (4.7) times that of system without modifications. Ten new Nusselt number correlations have been predicted for each type of modifications under investigation by using both Engineering Equation Solver (EES) software and the experimental data. Cost-Benefit analysis in terms of life cycle cost, net present value, cost-benefit ratio, and payback period have been conducted. From the analysis, it can be concluded that using (SW) system will save a significant amount of energy with a payback period of less than five years.

Numerical determination of the gas-side average heat transfer coefficients in the fin-and-tube heat exchanger

This paper presents the numerical method for the determination of the average heat transfer coefficient in fin-and-tube heat exchanger. The air side Nusselt number formulas are determined via the CFD simulation based method. The path of the gas flow across the inter-tubular space of this heat exchanger is complex, therefore the CFD simulations are used to determine the velocity distributions and the temperature field of air. The results of the numerical computations are compared with the Nusselt number formula based on the experimental data, which were obtained during the test of the car radiator. Good agreement between the numerical results and the results based on the measurements is achieved.

Study on Heat Transfer and Resistance Characteristics of H-Type Finned Tube

Three-dimensional numerical study is performed for heat transfer and resistance characteristics as well as comprehensive performance of two kinds H-type (single and double) finned tube. It is found that the heat transfer and resistance characteristics as well as comprehensive performance of H-type finned tube are influenced by the Reynolds number of gas. With the growth of Reynolds number, the air-side Nusselt number rises gradually and the heat transfer performance gets better and better, whereas the air-side Euler number drops step by step until close to a fixed value. The comprehensive performances of both single H-type finned tube and double ones are weaken progressively. When Reynolds number value is same, the convective heat transfer, pressure drop, air-side Nusselt number and Euler number of single H-type finned tube are bigger than those of double ones. The single H-type finned tube expression is much better than double ones in comprehensive performance and heat transfer.