Thermal performance analysis of fin-and-tube heat exchangers operating with airflow nonuniformity
Introduction
Fin-and-tube heat exchangers find extensive application in heating, ventilation, air conditioning and refrigeration systems, as well as in transport, process and power industries. Generally, heat exchangers are exposed to some degree of airflow nonuniformity, which can be caused by poor header design, the vicinity of the blower, adverse operating conditions, fouling and geometry imperfections on heat transfer surfaces, as found by Kitto and Robertson [1], Mueller and Chiou [2], Shah and Sekulić [3]. Airflow nonuniformity causes decrease of thermal performance, specifically a deterioration in the thermal effectiveness, along with a decrease of hydraulic performance, which manifests in a pressure drop increase.
The effects of airflow nonuniformity on the performance of fin-and-tube heat exchangers have been extensively studied using experimental, analytical and numerical methods. Generally, airflow nonuniformity causes effectiveness deterioration in the range between 5% and 15%, as found by Bahman and Groll [4], Hajabdollahi and Seifoori [5]. However, in the case of severe airflow nonuniformity, generated by faulty design of the inlet header, the effectiveness deterioration can be 25% or 30%, as found by Singh et al. [6]. When airflow nonuniformity induces refrigerant maldistribution on the tube side, the deterioration is up to 30%, as found by Yashar et al. [7].
Generally, the decrease of thermal performance depends on the degree of airflow nonuniformity and whether tube-side flow nonuniformity is also present. Mao et al. [8] carried out a numerical analysis on the effects of airflow nonuniformity on a condenser with R22 refrigerant. Three nonuniform airflow profiles at the inlet were studied: the parabolic, the saddle and the linear airflow profile. The parabolic airflow profile caused a 6% reduction in the condenser capacity and a pressure drop increase on both fluid sides, 32% on the refrigerant side and 135% on the air side. Likewise, the saddle and the linear airflow profiles caused a decrease of both thermal and hydraulic performance, although to a lesser extent than the parabolic profile. The respective reductions of the condenser capacity were 4% and 3% while the respective pressure drop increases were 13% and 35% on the refrigerant side, while 20% and 42% on the air side. Choi et al. [9] found that nonuniform airflow causes a reduction of the evaporator capacity by 8.7%, while tube-side refrigerant maldistribution reduces the capacity by as much as 30%. T'Joen et al. [10] performed experiments to assess the impact of airflow nonuniformity on the heat exchanger thermal performance. They found that nonuniform airflow profiles, such as the quadratic profile and the parabolic profile, cause a reduction in the overall heat transfer coefficient equal to 8.2% and 3.6%, respectively. Yashar and Domanski [11] used particle image velocimetry measurements to reveal airflow profiles in fin-and-tube heat exchangers. They saw that condensate collection trays and mounting brackets are the principal sources of airflow nonuniformity. Chiou [12] used finite difference method for a numerical study of the effects of airflow nonuniformity on crossflow plate heat exchangers. He concluded that the effectiveness deterioration depends on the heat exchanger nondimensional groups (number of transfer units NTU and heat capacity rate ratio C*), but also on the ratio of thermal resistances (R), the degree of airflow nonuniformity and the airflow regime.
Beiler and Kröger [13] developed a calculation method consisting of mass and energy conservation equations for every tube of fin-and-tube heat exchangers to analyze the local and the global effects of airflow nonuniformity. At first, they found that the effectiveness deterioration initially increases with the NTU, reaches a maximum value at NTU = 1.0–1.5 and subsequently decreases. On the second hand, they found that the effectiveness deterioration increases with the value of C*. This was confirmed by Ranganayakulu and Seetharamu [14] who found that the effectiveness deterioration is highest in heat exchangers with balanced heat capacity rates (C* = 1) and lowest in evaporators and condensers (C* = 0). Hajabdollahi et al. [15] used a teaching-learning optimization algorithm to account for the thermoeconomic performance of a crossflow plate-fin heat exchanger under flow maldistribution. Generally, they found that nonuniform profiles reduce the heat exchanger effectiveness and the thermoeconomic performance. However, interestingly, certain non-uniform profiles may lead to better thermoeconomic results than the uniform profile.
Guo et al. [16] arrived to a similar conclusion: inlet fluid flow nonuniformity can either deteriorate or enhance the heat transfer in crossflow heat exchangers, depending on the flow nonuniformity orientation. Ishaque et al. [17] carried out a numerical analysis of the effects of airflow maldistribution on the seasonal performance of heat pumps. Uniform airflow enhance the heat pump SEER and COP by 2–5% compared to nonuniform inlet profiles. Lee and Domanski [18] found that the capacity reduction in evaporators and condensers is higher for airflow nonuniformity than for refrigerant maldistribution. However, when airflow nonuniformity induces tube-side refrigerant maldistribution, the capacity reduction can be as much as 50%. In a related study, Lee et al. [19] developed an improved method for the analysis of fin-and-tube evaporators using R-22 or R–407C refrigerants and operating with airflow nonuniformity. The impact of three nonuniform airflow profiles was analysed: the convex, the concave and the inclined airflow profile. The reduction in the evaporator capacity was 6% for the concave profile, 5% for the convex profile and 3% for the inclined profile with respect to the reference capacity of the uniform airflow profile. Tu et al. [20] used PIV optical method to visualize the two-phase flow distribution in the header of a plate-fin heat exchanger. Perforated baffles or vane swirlers promote two-phase flow uniformity in the heat exchanger. Taler [21,22] developed mathematical models for tubular cross-flow and plate fin and tube heat exchangers. The approach includes subdivision of the heat exchanger tubes into finite volumes and applying water and air side heat transfer and fluid friction correlations for analysis of the heat exchanger thermal and hydraulic performance.
Recently, different compensation methods are being tested with the aim to reduce the impact of airflow nonuniformity. Kærn et al. [23] carried a numerical study of the performance of fin-and-tube evaporators with R-410A exposed to different sources of flow maldistribution. In the worst case, the reductions in the evaporator cooling capacity were: 16.4% for inlet vapor/liquid phase maldistribution from the distributor, 5.2% for maldistribution from the feeder tubes and 49.9% for airflow maldistribution. Superheat control in individual channels allows to recover large part of the capacity reduction. Kærn et al. [24] concluded that this compensation method recovers about 85% of the capacity reduction in the worst case of airflow nonuniformity. Bhaiyat et al. [25] detected a 60% thermal performance reduction in a louvered fin-and-tube heat exchanger subject to highly nonuniform airflow inside of an automotive climate control system. This reduction of thermal performance can be partially recovered by lattice porous heat exchangers, which outperforms conventional heat exchangers by up to 35%, as concluded by Bhaiyat et al. [26]. Vortex generators may be used to alleviate the impact of flow nonuniformity by increasing the air-side heat transfer, as reported by Chai and Tassou [27]. Various vortex generators shapes have been proposed such as delta wings and winglets [28], rectangular winglets [29], curved [30], trapezoidal and triangular winglets [31].
The present study is a follow-up to the previously published experimental and analytical investigations on the heat exchanger performance in nonuniform airflow [[32], [33], [34]]. Experimental investigations [32] pointed out that the effectiveness deterioration of fin-and-tube heat exchangers subjected to low and moderate degrees of airflow nonuniformity was between 5% and 10%, while the pressure drop increase was between 10% and 20%. However, experimental investigations performed for severe degrees of airflow nonuniformity [33], showed that the effectiveness deterioration was as much as 30% and the pressure drop increase was up to 120%. Generally, different heat exchangers perform differently when exposed to airflow nonuniformity. Experimentally validated calculation method [34] proved that the exact value of the effectiveness deterioration depends on the heat exchanger dimensionless groups (number of transfer units NTU and heat capacity rate ratio C*) as well as on the tube circuity design and the orientation of the airflow nonuniformity.
Airflow nonuniformity, if not accounted for, leads to discrepancies between the predicted and the observed thermal and hydraulic performance of heat exchangers. The above referenced studies are mainly limited to one-dimensional nonuniform air profiles, or to particular heat exchanger types and operating conditions. This paper makes a step forward by introducing a new and general method, along with its experimental validation, for the thermal performance analysis of fin-and-tube heat exchangers operating with airflow nonuniformity. The method has broad capabilities and can reveal the effects of one-dimensional and two-dimensional entrance airflow nonuniformities on the heat exchanger thermal performance, as well as to understand the mutual impact of tube circuitry design and airflow nonuniformity orientation.
Section snippets
Physical problem
The physical problem represents fin-and-tube heat exchangers containing copper tubes and flat plain aluminum fins. Two fin-and-tube heat exchangers, with different heat transfer surface geometries have been analysed. The first fin-and-tube heat exchanger has a total of 38 copper tubes in 3 tube rows and 7 tube circuits. The inner and the outer tube diameters are 14.8 mm and 15.9 mm, respectively. The longitudinal and the transversal tube pitches are 30 mm and 60 mm. The heat exchanger contains
Methodology
General thermal design methods assume uniform airflow at the heat exchanger entrance, which leads to discrepancies between predicted and observed thermal performance in heat exchanger subject to airflow nonuniformity. The thermal effectiveness deterioration from airflow nonuniformity has been assessed using a tube element calculation method, validated against experimental tests.
The test line
The thermal performance of fin-and-tube heat exchangers operating with airflow nonuniformity, calculated with the tube element method, was evaluated against experimental data. Measurements were carried out in the Laboratory for Thermal Measurements at the University of Rijeka, Faculty of Engineering. The heat exchanger test section is shown in Fig. 5 and the scheme drawing of the test line is shown in Fig. 6. The test line comprises an open wind tunnel, a closed water loop, the heat exchanger
The heat exchanger effectiveness for uniform airflow
The reference effectiveness of the two test fin-and-tube heat exchangers operating with uniform airflow is obtained using the tube element method for uniform airflow, section 3.1. Two ε-NTU solutions exist: the first with air being the weaker stream and the second with tube side fluid being the weaker stream. The tube element method can predict both solutions using the appropriate relationship between local and global nondimensional groups, expressions (2) and (3). In this study, all ε-NTU
Conclusions
This paper assessed the thermal performance deterioration of fin-and-tube heat exchangers operating in airflow nonuniformity. The most important conclusions obtained from the results of the tube element method and its experimental validation are summarized below:
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the tube circuitry design plays a major role in the heat exchanger effectiveness, both for uniform and nonuniform airflow conditions;
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the size of effectiveness deterioration depends mostly on the heat exchanger operating point (C* and NTU
Credit author statement
Paolo Blecich: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Resources, Software, Validation, Visualization, Writing – original draft, Writing – review & editing. Anica Trp: Funding acquisition, Project administration, Supervision, Conceptualization, Methodology, Formal analysis, Investigation, Resources, Validation, Visualization, Writing – review & editing.Kristian Lenić: Supervision, Conceptualization, Methodology, Formal analysis, Investigation, Resources,
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
This work has been supported in part by Croatian Science Foundation under the project HEXENER (IP-2016-06-4095) and in part by the University of Rijeka under the project number “uniri-tehnic-18-69”.
Nomenclature
- A
- heat transfer surface area, m2
- C
- heat capacity rate, W K−1
- C*
- heat capacity rate ratio, Cmin/Cmax,
- c
- specific heat capacity, J kg−1 K−1
- d
- diameter, mm
- Ffnu
- combined flow nonuniformity factor,
- g
- flow nonuniformity factor,
- h
- heat transfer coefficient, W m-2 K-1
- k
- thermal conductivity, W m−1 K−1
- ṁ
- mass flow rate, kg s−1
- N
- number,
- NTU
- number of transfer units,
- Pr
- Prandtl number,
- Q̇
- heat transfer rate, W
- R
- thermal resistance, m2 K W−1
- Re
- Reynolds number,
- S
- degree of airflow nonuniformity,
- sf
- fin pitch, mm
- T
- temperature, K
- average
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