Experimental investigation on particle deposition characteristics of wavy fin-and-tube heat exchangers

Highlights

• A visual experimental rig was designed to observe particle deposition on heat exchanger.

• A weight method including a moveable analytical balance was adopted to measure weight of deposited particles on heat exchanger.

• Particle deposition on a wavy fin-and-tube heat exchanger mostly occurs on the leading edge of fins and the front part of tubes.

• Decreasing fin pitch or increasing particle concentration may lead to more particle deposition.

• Increase of air velocity promotes particle deposition at lower air velocity while restrains particle deposition at higher air velocity.

Abstract

In this study, the particle deposition characteristics of wavy fin-and-tube heat exchangers were experimentally investigated. A visual experiment apparatus was designed to observe the distribution of particles on the wavy fin surface and to measure the weight of deposited particles. Experimental conditions cover fin pitch over a range from 1.6 mm to 3.2 mm, particle concentration over a range from 80 kg m⁻³ to 280 kg m⁻³ and air velocity over a range from 1 m s⁻¹ to 3 m s⁻¹. The observation results show that the particles mostly deposit on the leading edge of fins as well as the front part of tubes. The weight measurement results show that fin pitch and particle concentration have a monotonic effect on particle deposition; the maximum particle deposition weight per unit area increases by 13.1% and 6.2% respectively as fin pitch decreases and particle concentration increases; while air velocity has a complicated effect on particle deposition, the maximum particle deposition weight per unit area firstly increases by 6.8% and then decreases by 10.9% as air velocity increases.

Introduction

Fin-and-tube heat exchangers are the most commonly used heat exchanger types in air-conditioning systems. The fin types can be categorized into plate fins [1], [2], wavy fins [3], [4], slit fins [5], [6] and louver fins [7], [8], [9]. Slit fins and louver fins are widely used in indoor unit heat exchangers of air conditioners because they have a lot of slotted structures to increase the heat transfer efficiency. However, these fins are seldom used in outdoor units of air conditioners because the slotted structures are easily blocked by dust particles in an outdoor ambience. Plate fins and wavy fins are the two types of fins that are commonly used in outdoor unit heat exchangers. The advantages of plate fin come from its simplicity and low air-side pressure drop [10], but the heat transfer coefficient is generally lower than wavy fin [11]. Wavy fin shows larger heat transfer performance since it can lengthen the flow path of the airflow and cause better air flow mixing [12], [13]. Considering of the effective improvement of thermal performance and the significant reducing of size and weight of outdoor unit heat exchangers, wavy fin-and-tube heat exchanger has better overall heat transfer performance. Thus wavy fin-and-tube heat exchangers become the most common fin-and-tube heat exchangers in the outdoor units of air conditioners with high performance.

The performance of a wavy fin-and-tube heat exchanger at an outdoor environment will decrease after long-term operation because dust particles involved in the outdoor air may partly deposit on fins and tubes when the dusty air flows through the heat exchanger. Nowadays, many CFD simulation researches have been carried out to improve the performance of wavy fin-and-tube heat exchanger by optimizing geometric parameters such as fin pitch, wavy angle and tube arrangement [14], [15], but none have taken the effect of dust deposition into consideration. The thermal conductivity of dust is far less than those of fins and tubes, and the accumulated particulate fouling on the fins can reduce the air flow area, then dust deposition may increase air side thermal resistance and pressure drop, which results in a significant deterioration of the long-term performance of heat exchangers [16], [17]. For example, Ahn et al. [18] found that the heat transfer capacity of indoor heat exchangers may decrease by 10~15% and the air side pressure drop may increase by more than 44% after 7 years. For outdoor heat exchangers, the long-term performance may decrease even more because the air cleanliness in the outdoor is worse than that in the indoor, so it is meaningful to investigate the dust deposition characteristics on wavy fin-and-tube heat exchangers.

The effect of dust particles on a wavy fin-and-tube heat exchanger results from the forming of a fouling layer on the fin and tube surfaces. The dust particles are firstly carried onto the heat exchanger surface due to transport mechanisms such as inertial impact, Brownian movement and turbulent diffusion [19], and then deposit on the heat exchanger by the impact behaviors of the incident particles hitting the fin surfaces and tube surfaces [20]. The deposited particles will continually accumulate on the heat exchanger surface; meanwhile, the growing fouling layer may be blown away by the dusty air flow when it grows to a certain thickness [21]. The ultimate status of dust particle deposition is determined by the coupling effect of two factors, i.e., the dusty air flow characteristics and the heat exchanger structure.

The dusty air flow characteristics on particle deposition are affected by the physical properties of dust and the air flow conditions. The effect of physical properties of dust, covering the size ranges of dust particles and the constituents of fouling materials, has been studied. The detection results of the size ranges of dust particles demonstrated that the discrete size range of deposited particles are generally from 5 to 100 µm in places such as restaurants, offices and hotels [22], while from ultrafine to 10 µm in places requiring high air quality like aircraft cabin [23]. The constituents of fouling materials include not only dust particles but also different types of fibers (i.e., clothes fibers, paper scraps, pet hair and fur) [22], and the existence of fibers increases the particle deposition weight due to the filtration function of fibers [24]. However, the effect of air flow conditions (i.e. air velocity and dust concentration) on particle deposition has not been reported in open literatures.

The effect of heat exchanger structure on particle deposition results from the tubes and the fins. The influence factors from the tubes include the tube pitch and the number of tube rows. Small tube pitch can reduce particle deposition [25], [26], [27], and Han et al. [28] recommended a tube pitch value of 2 mm by simulating the particle deposition rate on the tube bundle heat exchanger with a 6-row tube arrangement. The increase of the number of tube rows is beneficial to particle deposition, and the relationship between the deposition rate and the number of tube rows is nonlinearity [29], [30]. The influence factors from the fins include the fin pitch, the fin type and the wettability of fins, in which the last two factors have been investigated by the existing researches. The effects of four types of fins on particle deposition were studied, covering slit fins [31], [32], [33], louver fins [33], [34], [35], wavy fins [32], [33], [34], [35], [36] and plane fins [32], [34], [36], [37], [38]; the results showed that the dust particle deposition on the wavy fins is more serious than that on the plane fins while less obvious than that on the slit fins or louver fins. The effect of the wettability of fins includes wet conditions [31], [32], [35], [36], [37], [39] and dry conditions [32], [34], [37], [38]; the results showed that much more dust particles are prone to deposit on the fin surfaces under wet condition. However, no study has been reported concerning the influence of the fin pitch on dust particle deposition.

As mentioned above, air velocity, particle concentration and fin pitch are three key factors affecting particle deposition but their effects on the particle deposition characteristics on wavy fin-and-tube heat exchangers have not been illustrated in the existing publications.

In order to sufficiently know the particle deposition characteristics on the wavy fin-and-tube heat exchangers, the experimental investigation will be performed in this study, considering the effects of dusty air flow characteristics (air velocity and particle concentration) and the effect of heat exchanger structure (fin pitch) on the weight of particles deposited on wavy fin-and-tube heat exchangers.