Hydraulic performance and deposition enhancement of ultrafine particles for in-duct twisted tapes under stationary and rotating conditions

Journal Pre-proofs Hydraulic Performance and Deposition Enhancement of Ultrafine Particles for In-duct Twisted Tapes under Stationary and Rotating Conditions Huihui Zhang, Xin Jin, Sunday Segbenu Nunayon, Alvin Chi-keung Lai PII: DOI: Reference:

S1359-4311(19)34234-6 https://doi.org/10.1016/j.applthermaleng.2019.114519 ATE 114519

To appear in:

Applied Thermal Engineering

Received Date: Revised Date: Accepted Date:

19 June 2019 12 September 2019 8 October 2019

Please cite this article as: H. Zhang, X. Jin, S. Segbenu Nunayon, A. Chi-keung Lai, Hydraulic Performance and Deposition Enhancement of Ultrafine Particles for In-duct Twisted Tapes under Stationary and Rotating Conditions, Applied Thermal Engineering (2019), doi: https://doi.org/10.1016/j.applthermaleng.2019.114519

This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

© 2019 Published by Elsevier Ltd.

Hydraulic Performance and Deposition Enhancement of Ultrafine Particles for In-duct Twisted Tapes under Stationary and Rotating Conditions Huihui Zhanga, Xin Jina, Sunday Segbenu Nunayona, Alvin Chi-keung Laia* aDepartment

of Architecture and Civil Engineering, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China

Abstract Sustainable and energy-efficient air-conditioning systems require high-performance filtration systems with low air resistance. Experimentally, we investigated different diameters of both stationary and rotating twisted tapes (TTs) duct-inserts to characterize their hydraulic performance and level of ultrafine particles (UFP) deposition. It is an innovative study using an active approach (rotating TTs) for particle removal in the ventilation duct. Friction factors and particle deposition velocities were calculated both for empty ducts and for ducts with TT inserts in stationary and rotating conditions under constant pumping power. In addition, the overall particle deposition-hydraulic performance index was established to quantify the effectiveness of TTs under both laminar and turbulent airflows. The results showed a marginal difference in friction factor between stationary and rotating TTs under turbulent flow. The rotating TTs yielded a higher UFP deposition velocity than stationary TTs, peaking at 7.3 times compared to the deposition velocity for empty duct for particles smaller than 20 nm. Moreover, the highest performance index of a rotating TT could reach 6.5 for particles smaller than 20 nm at a Reynolds number of 10,000. Overall, a rotation or an increase in diameter of TTs both could improve the particle deposition performance. Notably, this study is the first to develop a filterless system for the removal of particles in the ventilation duct. Using in-duct rotating TTs would provide a solution for developing high filtration devices at less pressure penalty or smaller fanning energy consumption.

Corresponding author. E-mail address: [email protected] (A.C.K. Lai). *

1

Keywords: rotating twisted tape, ultrafine particles, deposition enhancement, friction factor, duct flow

Nomenclature ∆𝐶𝑖

Particle concentration difference m1 and (#/cm³)

Cup,i

Cd,i

Correlated values

m2

Upstream particle concentration k1 and (#/cm³)

k2

Downstream particle

B

Correlated values

Brownian diffusion coefficient

concentration (#/cm³) Dh

Hydraulic diameter (m)

Greek symbols

Vd ratioi

Deposition velocity ratio

𝜂

Particle deposition performance ratio

LOi

Particle removal percentage

L

Length of the surface concerned 𝜌𝑎

𝑣

Kinematic viscosity (m2/s) Air density (kg/m3)

(m) U

Mean air velocity (m/s)

Subscripts

Vd,,i

Deposition velocity (m/s)

i

f

Friction factor

Abbreviations

ΔP

Pressure drop (Pa)

UFP

Re

Reynolds number

1 Introduction 2

Particle bin or particle size group

Ultrafine particles

A considerable body of epidemiologic and toxicologic evidence has shown the severe health effects of inhaling ultrafine particles (UFP) due to their high deposition fraction in the alveolar region, the endotoxin content, and possible transportation into the circulatory system [1-2]. These UFP have diameters of less than 100 nm and are generated by a variety of activities, such as cooking, smoking, toasting, incense burning, housecleaning, printer printing, and vehicle emissions [2-4]. Also, as the particle size decreases, the fraction of UFP that are deposited while breathing is expected to increase [5]. Studies [4,6] have reported that indoor UFP sources give more contributions of personal exposure than the outdoor sources. Because people spend most of their time indoors, it is necessary to use effective engineering control strategies to remove UFP from the air and ensure good indoor air quality (IAQ) . The general strategy for providing good IAQ is the use of heating, ventilation, and air-conditioning (HVAC) systems in buildings. HVAC systems are typically composed of a series of ductwork that can act as an “intermediary” between outdoor and indoor environments. They do not only provide thermal comfort for the occupants but also acceptable IAQ. Filtration is needed in HVAC systems to remove particles from both outdoor and indoor sources. While the removal efficiencies of MERV (Minimum Efficiency Reporting Value) 6 filters for most UFP are less than 10% [7], the MERV 8 filters’ efficiency for removing 100nm particles is nearly 0 [8]. The use of medium-grade filters, MERV 6-9, are common in most commercial buildings, but studies have shown that these filters are less effective for removing airborne microorganisms such as small bacteria and viruses [9,10]. In contrast, high-efficiency fibrous filters that featuring very fine closely packed glass fibers are often used for applications in special situations like clean rooms. they are not currently installed in general-purpose applications due to their excessively high pressure drop and high operating cost [10,11]. From the perspective of energy consumption, the fan energy required to overcome the flow resistance accounts for 30% of an HVAC system’s average energy consumption [12]. Therefore, 3

maintaining good IAQ and achieving substantial energy savings in buildings require a solution that can remove the UFP from the air in an energy-efficient way. It has been shown that rough surfaces or roughness elements on walls can reduce the thickness of the viscous sub-layer near the wall and provide sites for particle impaction, which results in further enhancement of particle deposition [13-14]. For example, a previous study conducted by our group investigated the effect of ventilation ducts outfitted with transverse ribs and pseudo-3D elements on the deposition of particles between 0.7 and 7.1μm [14], and the results showed that the deposition velocity of a ribbed surface can reach seven times that of a smooth surface. Also, existing numerical studies have investigated the effects of variable crosssections [15], duct bends [16], air velocity [17-18] and thermophoretic force [19-20] on particle deposition. Besides, many works have investigated the characteristics of fouling or particle deposition in heat exchangers [17-22]. For example, Zhan et al. [17] experimentally studied the effects of air velocity, particle concentration, and fin pitch on the particle weight in wavy finand-tube heat exchangers. They observed that decreasing fin pitch or increasing particle concentration could promote particle deposition. Han et al. [20] also conducted numerical simulations to investigate the deposition of particles between 1 and 50 μm in heat exchange pipelines. Their observations showed that the influence of thermophoresis on the deposition of small particles was greater than that of large particles. However, these studies were all conducted for particle sizes larger than 0.5 µm. Because the size of UFP is very small, their transport and deposition mechanisms differ greatly from that of large particles [15, 23-24]. The results could be more informative if UFP and the pressure penalty were both taken into consideration. Many factors influence the particle deposition process, including the particle size, Reynolds number, surface roughness, thermophoresis, particle charge, turbophoresis, and gravity [24-25]. With respect to particles of various sizes, studies have shown that the results 4

of particle deposition velocity into three regimes: (1) diffusional deposition, (2) diffusionimpaction, and (3) inertia-moderated regimes [23-24]. Due to the very small particle size, diffusion is commonly considered the dominant mechanism for the deposition of UFP. In this case, the deposition mechanism would fall into the first diffusion regime [26]. In the diffusional deposition category, turbulent diffusion or eddy diffusion in the core flow region and Brownian diffusion in a thin boundary layer both affect the transport process of particles toward the deposition wall or surface [23,27], but for larger particles in the diffusion-impaction or inertiamoderated regimes, particle inertia is also an important mechanism for their deposition [15]. One of our recent studies investigated the deposition of UFP using a riblet surface, which also can be considered as a rough surface [28]. The results agreed with the conclusions in the literature, showing that an effective enhancement of UFP deposition is possible via an increase in turbulent diffusion [23]. Many approaches can be used to increase turbulence. One of the useful, extensively studied and well-known passive approaches is the use of twisted tapes (TTs) for enhanced heat transfer (EHT)[29-31]. The mechanism responsible for the enhancement is the periodic interruption of the boundary layers and the generation of turbulence and superimposed vortex motion. Studies have remarked that a reduction in the twisted ratio [32] and a greater thickness [33] could increase the convective heat transfer coefficient of TTs. Also, a study reported that perforated TTs demonstrated better performance than solid TTs [34]. Various studies have also investigated TTs with different geometries and modifications for EHT, such as multiple TTs [35], short TTs [36], square-cut TTs [37], helical TTs [38], cross hollow TTs [39] and regularly spaced TTs with rods and spacers [40]. The results indicated that secondary flow (swirl flow) induced by TTs is the most dominant factor in the enhancement of heat transfer using the insert technology. Besides, several studies have proposed the rotation of TTs as an active approach

5

for enhancing thermal performance[31,41-42]. The results showed that thermal performance was higher for rotating TTs than stationary TTs. However, studies on the application of TTs for the deposition of UFP are rare, except for a previous pilot study conducted by our group [25], in which the results showed that placing TTs in the duct system could enhance the particle deposition velocity. The length of the tested TT in this earlier work was 285 mm long. Thus, because of the high potential of its application to improve IAQ and develop high-efficiency air purification equipment, it is worthwhile to further investigate and quantify the enhancement of UFP deposition by TTs. Therefore, in this study, we proposed the use of rotating and stationary TTs to enhance particle deposition. It is the first study using rotating TTs to replace fibrous filters for particle removal in the ventilation duct. The objective of this work is to investigate the effects of the diameter, rotation, and stationary features of TTs on hydraulic performance and particle deposition. Various particle sizes (from 0 nm to 150 nm) and Reynolds numbers (Re) ranging from 300 to 10,000 were tested. In addition, to evaluate the performance of ducts with TT inserts, the compressive performance index of particle deposition was first deduced and established for the laminar flow and turbulent flow conditions. As more scientific and toxicological evidence suggests the severe health effects of inhaling of UFP, there is a growing demand for high filtration of UFP, and the need for more sustainable and energy-efficient air-conditioning systems. Using rotating or stationary TTs would provide a solution for developing high filtration devices at less pressure penalty or smaller fanning energy consumption. 2 Materials and Methods 2.1 Experimental System Design

6

(a)

(b) Fig. 1 (a) Schematic of the experimental apparatus used in this work. (b) Schematic of the test TT.

To investigate the effects of rotating and stationary TTs on the hydraulic performance and deposition of UFP in ducts, a scaled air duct test system was designed and built in an airconditioned laboratory. The 2.92-m-long square duct was positioned horizontally across the airstream, which was made of transparent Perspex with a cross-section of 15 mm width × 15 mm height. The schematic of the experimental test system is shown in Fig. 1. The test section is the 1.33-m-long duct between points B and C, as shown in Fig. 1 (a). The exit length between points C and D is 250 mm long. The entry length is 1340 mm, as shown in Fig. 1 (a) for the 7

duct between points A and B. It is heavily outweighed by a factor of 30 times with respect to the hydraulic diameter of the duct, and it thus ensures that the airflow is fully developed in the test section [35]. The experimental setup consists of a compressor, nebulizer, neutralizer, dryer, mixing chamber, high-efficiency particulate air filter, centrifugal fan, and scanning mobility particlesizer spectrometer (SMPS) system. The function of the dryer and neutralizer was to eliminate the effects of particle charge and droplets on particle deposition. As shown in Fig. 1, the compressor supplied compressed air into the 24-jet nebulizer (Collision Nebulizer, BGI). The nebulizer operated at a constant air pressure of 68.95 kPa (10 psi). The particles were produced by dissolving 0.5 g of NaCl in 900 ml of deionized water, and compressed air was imparted on the solution by passing it through the nebulizer. The NaCl solution was aerosolized by the nebulizer, and the output was transported to the dryer to remove droplets from the air and to the neutralizer to neutralize the particles before it was injected into the inlet of the duct system to mix with the laboratory air. The laboratory air was maintained at 23°C ± 2°C with a relative humidity of 55% ± 5%. The installed high-efficiency particulate air (HEPA) filter at the mixing chamber aided the cleaning of the input air. A centrifugal fan was used to induce airflow in the duct, and could be adjusted for various airflow velocities with a variable speed controller. The airstream velocity in the duct was measured with a portable anemometer (Kanomax A004) inserted through point B (see Fig. 1 (a)). The sensor of the anemometer was placed perpendicular to the airstream to ensure proper airspeed readings. The accuracy of the anemometer sensor was ±3% of the readings or 0.015 m/s, whichever was greater. The measurement was repeated for various airflow velocities during the experiment. The tested velocity in the duct was adjusted between 0.3 and 10 m/s, which corresponded to Reynolds numbers (Re) between 300 and 10,000, ranging from laminar to turbulent in terms of airflow conditions. 8

To evaluate the hydraulic performance for both the empty duct and the duct with built-in TTs, the pressure drop must be measured between the front and back of the test section. Therefore, the differential static pressure at upstream and downstream locations across the test section was measured with an IAQ meter (Testo 480) at the pressure traps (at measuring points B and C shown in Fig. 1(a)). The IAQ meter has an internal piezoresistive sensor with an accuracy of 1% for measurement of differential pressure between 0 to 2500 Pa. As indicated in the pressure drop measurements, the tested pressure drop in the experimental system ranged from 1 to 240 Pa. For UFP deposition experiments, the particle concentration was measured at upstream and downstream test points denoted as locations B and C, respectively, in Fig. 1 (a). The SMPS system, consisting of a long differential mobility analyzer (DMA, Model 3081, TSI) and a condensation particle counter (CPC, Model 3775, TSI), were used to measure the size-resolved particle concentration from 5.94 to 150 nm. Furthermore, SMPS has a great advantage in measuring the concentration in bins instead of the total concentration. To facilitate the analysis of the experimental data, the results were placed into four groups: D < 20 nm (group 1), D = 20 to 50 nm (group 2), D = 50 to 100 nm (group 3), and D = 100 to 150 nm (group 4). According to the manufacturer’s manual, the accuracy of particle concentration counted by CPC is ±10% when the number of concentration is lower than 5 ×104 particles/cm3. However, for a higher particle concentration (i.e., <107 particles/cm3), the accuracy of the CPC is ±20%. To achieve high accuracy, the particle concentrations we measured were less than 5 ×104 particles/cm3. 2. 2 Details of Rotating and stationary TTs In this study, we tested four TT conditions: rotating and stationary TTs with diameters of 8 and 10 mm. All TTs were fabricated with stainless steel and had a length of 1 m. A schematic of the TT is shown in Fig. 1 (b). Their twist ratios (ratio of pitch to diameter; H/D) were all 3.

9

Stationary 10-mm and 8-mm TTs were placed in a predetermined location between point B (upstream) and point C (downstream) inside the ductwork (see Fig. 1 (a)). For the rotating TT, the TT insert setup was modified to allow for rotation along the streamwise direction. The TTs’ rotation speed was measured with a non-contact digital tachometer (TachoHiTester3404, Hioki) with an accuracy of 0.01 r/min by measuring the light reflected from the reflective tape attached to the TT surface. A DC-driven motor was used to drive the rotation movement of the TTs, which connected the TTs with a very small rod. The voltage was set as 10 V for the rotating TTs setup, hence the power consumed by the motor can be calculated as the product of the input current and the corresponding voltage. The power consumed by the motor was set to remain constant at 0.2 W. This results in a rotation frequency of 50 rpm in the motor. It is worthwhile to note that the motor and the DC power were both placed outside the duct to avoid any disturbance of the duct airflow. 2. 3 Experimental Procedure First, pressure drop and particle deposition measurements were carried out under various airspeeds for the empty duct scenario. The fan speed was gradually adjusted and the anemometer reading monitored until the airflow velocity reached to the target airflow speed. Before the particle deposition experiments, the pressure drop between points B and C was recorded at each airspeed. The NaCl solution was then nebulized and fed into the ductwork system for 5 minutes before sampling to ensure that the concentration output had stabilized. The upstream concentration was scanned first, then, the sampling tube was inserted at the downstream position to scan the downstream aerosol concentration. To achieve reliable and credible measurements, the sampling tube was put back at the upstream location, and the particle sampling procedure was repeated at least nine more times. After one velocity setting was finished, the fan power was adjusted to another level with a variable speed controller, and the procedure was repeated. 10

After finishing the experiments for the empty duct, the measurements for the stationary and rotating TTs were conducted under the same pumping or fanning power conditions as the empty duct for each recorded airspeed. The sampling procedure for measurement of the ducts fitted with stationary and rotating TTs was the same as for the experiments with an empty duct. 2. 4 Data reduction and establishment of the performance index In this work, pressure drop tests and particle deposition experiments were conducted to analyze the hydraulic characteristics and particle deposition performance of the TTs. The measurements for the empty duct and for the duct with TT inserts under stationary and rotating conditions were carried out with Reynolds numbers from 300 to 10,000. The formula of Reynolds number is given by

𝑅𝑒 =

𝑈𝐷ℎ 𝑣

(1)

where U is mean air velocity in the duct. Dh and 𝑣 are the hydraulic diameter of the duct and the air kinematic viscosity, respectively. The friction factor f was calculated from the pressure drop, ΔP, across the upstream and downstream of the air duct. It can be written as [35]:

𝑓=

2∆𝑃𝐷ℎ 𝜌𝑎𝑈2𝐿

(2)

in which L is the length of the surface concerned. Here, L equals the length of the test section, namely 1.33 m. For particle deposition characteristics, the particle loss percentage, deposition velocity, and enhanced deposition velocity ratio were determined. First, the concentration difference of the particles for each size bin i is given by ∆𝐶𝑖 = 𝐶𝑢𝑝,𝑖 ― 𝐶𝑑,𝑖 11

(3)

Where ∆𝐶𝑖 is the particle concentration difference of bin i, and Cup,i and Cd,i are the concentrations of bin i upstream and downstream, respectively. The particle loss percentage or removal efficacy (LOi) for each bin i can be expressed as 𝐿𝑂𝑖 =

∆𝐶𝑖 𝐶𝑢𝑝,𝑖

× 100% .

(4)

The particle deposition velocity was commonly regarded as a quantifying parameter in the deposition process [14-15,28]. In this work, LOi was measured by Eq. (4) and was based on the mass balance of particles. It is hence related to the deposition velocity (Vd,i) by the following theoretical equation [44]:

( 𝐿𝑂 = 1 ― 𝑒

).

(5)

ln (1 ― 𝐿𝑂𝑖)

(6)

―4𝐿𝑉𝑑,𝑖 𝐷ℎ𝑈

𝑖

Equation (5) can then be rewritten as: 𝑉𝑑, 𝑖 = ―

𝐷ℎ𝑈 4𝐿

After calculating the deposition velocity of the individual bin, the deposition velocity for each group was determined by averaging the Vd,i for those bins within the group. The deposition velocity ratio (Vd ratio) was calculated as the ratio of the particle deposition velocities in the duct fitted with TTs to the particle deposition in the empty duct system. It is given as: 𝑉𝑑 𝑟𝑎𝑡𝑖𝑜 =

𝑉𝑑,𝑇𝑇 𝑉𝑑,𝑒𝑚𝑝𝑡𝑦

(7)

where Vd,TT and Vd,empty represent the particle deposition velocity for ducts with TT and empty ducts, respectively. The enhancement in particle deposition by the duct or pipe with inserts is commonly accompanied by a rise in pressure drop. Therefore, to evaluate the performance of various inserts in the duct or pipe flows, it is useful to simultaneously consider the effects of particle deposition enhancement and the flow drag.

12

As indicated in the studies [29,45] of EHT, an empty tube without inserts is often considered the benchmark for comparison with various inserts. Because the particle deposition velocity and friction factor are both affected by the Reynolds number in air flows at a high level of sensitivity, a meaningful comparison of deposition and pressure drop characteristics between the empty duct and the duct with inserts can be made at the same pumping power. Equal pumping conditions for the plain tube and for tubes with various inserts have been widely used for performance evaluation in heat transfer studies [29,39,45]. Thus, in this work, we used this same comparative method and tested the empty duct and ducts with TTs at identical fanning power. For the establishment of a performance index in particle deposition, we assume that the equivalent diameter, cross-sectional area, and dimensionless parameter of the duct with inserts are the same as that of an empty duct. The same assumptions have commonly been adopted in the performance evaluation of the enhancement of heat transfer [45]. Thus, under the same pumping condition, the overall performance index, η, for UFP deposition can be presented as: 𝑉𝑑, 𝜂=

𝑇𝑇

𝑉𝑑, 𝑒𝑚𝑝𝑡𝑦 𝑚1/(3 + 𝑚2)

( ) 𝑓𝑇𝑇

(8)

𝑓𝑒𝑚𝑝𝑡𝑦

The detailed derivation steps of equation (8) can be found in the supplementary material (S1). Here m1 and m2 are the correlated powers of Reynolds number for particle deposition velocity and friction factor for empty duct, respectively, and are defined in the following equations: 𝑉𝑑(𝑅𝑒) = 𝑘1𝑅𝑒𝑚1 𝑓(𝑅𝑒) = 𝑘2𝑅𝑒𝑚2

(9) (10)

For equations (9) and (10), it is assumed that the deposition velocity of the UFP and the friction factor of the empty duct can both be correlated with the Reynolds number. Shimada et 13

al. [44] studied the deposition velocity of monodisperse particles between 10 and 40 nm in the pipe air flows with Reynolds numbers between 100 and 10,000. They correlated the particle deposition velocity with the Reynolds number, the Brownian diffusion coefficient (B), and the pipe diameter via numerical studies by the following equation: 𝑉𝑑 =

2.4 × 10

―4

𝑅𝑒 𝐷ℎ

0.92 0.67

𝐵

(11)

Therefore, 0.92 can be considered as an estimate of m1. In contrast, to compute the value of m2, the Poiseuille’s law and Blasius correlation for laminar and turbulent flows respectively denote that: 𝑓=

64 𝑅𝑒

𝑓𝑜𝑟 0 < 𝑅𝑒 < 1500

𝑓 = 0.316𝑅𝑒 ―0.25 𝑓𝑜𝑟 3000 ≤ 𝑅𝑒 ≤ 5 × 106

(12) (13)

Hence, the value of m2 equals -1 for laminar flow, or -0.25 for turbulent flow. In laminar flow, the value of 𝑚1/(3 + 𝑚2) equals 1/2. The overall performance index, η, for UFP deposition is 𝑉𝑑, 𝜂=

𝑇𝑇

𝑉𝑑, 𝑒𝑚𝑝𝑡𝑦 1/2

( ) 𝑓𝑇𝑇

.

(14)

𝑓𝑒𝑚𝑝𝑡𝑦

In turbulent flow, the value of 𝑚1/(3 + 𝑚2) equals 1/3, the η is shown as follows: 𝑉𝑑, 𝜂=

𝑇𝑇

𝑉𝑑, 𝑒𝑚𝑝𝑡𝑦 1/3

( ) 𝑓𝑇𝑇

𝑓𝑒𝑚𝑝𝑡𝑦

3 Results and Discussion 3.1 Friction factor

14

(15)

Pressure drop measurements were carried out with various Reynolds numbers for both empty duct and ducts fitted with TTs. Based on equation (2), friction factors were obtained in laminar and turbulent flow regimes. Fig. 2 depicts variations in the friction factor with the Re for the empty duct. It is observed that f decreases as the Re increases. With an increase in the Reynolds number from 300 to 1220, the experimental friction factor in laminar flow changed from 0.209 to 0.053. It also changed from 0.037 to 0.031 in turbulent flow as the Reynolds number increased from 5000 to 10,000. As shown in Fig. 2, the experimental results of the friction factors of an empty duct with various Reynolds numbers were compared with predictions by Poiseuille’s law for laminar flow (equation (12)) and the Blasius correlation for turbulent flow conditions (equation (13)). The predicted friction factors are depicted with two solid lines in the Fig. 2. At Reynolds numbers of 300, 1220, 5000, and 1000, the differences between the experimental and predicted results were 2.10%, 1.11%, 1.38%, and 0.61%, respectively. We notice that there is a good agreement between the experimental results and the predicted values for the friction factor in both laminar and turbulent flow conditions.

Fig. 2 Variation of friction factor with the Reynolds number for empty duct 15

The effect of using either rotating or stationary TTs on the pressure drop is presented in terms of the friction factor with Reynolds number in Fig. 3. For better illustration and clarity of each point, we divided the wide range of Reynolds numbers into a laminar region and turbulent region. In the Fig. 3, the friction factors were compared under the identical pump power. As expected, the TT inserts result in a higher drag penalty than the empty duct because the TT acts as a kind of turbulator that interrupts the boundary layer and induces the generation of turbulence in the duct, which results in an increase in the friction factor. At a Reynolds number of 300, the calculated friction factors were 0.355, 0.334, 0.292, and 0.251 for ducts fitted with 10-mm rotating, 10-mm stationary, 8-mm rotating, and 8-mm stationary TTs, respectively. Compared with an empty duct, the maximum increases in flow resistance occurred in the laminar flow, and the values were 131%, 126%, 71%, and 57% for the 10-mm rotating, 10-mm stationary, 8-mm rotating, and 8-mm stationary TTs, respectively. It was observed that the 10mm rotating TT had highest friction factor among the four ducts outfitted with TTs, followed by the 10-mm stationary TT and the 8-mm rotating TT, whereas the friction factor in the duct outfitted with the 8-mm stationary TT had the lowest values for all tested Reynolds numbers. We were interested to note that no significant increase was seen in the friction factor of the rotating TTs compared with that of the stationary TTs. Especially in turbulent flow, the friction factors of the rotating TTs were only slightly higher than those of the stationary TTs. From an analysis of the data shown in Fig. 3, similar decreasing trends with Re were observed for various TTs in both laminar and turbulent flow regimes. Inferring from Fig 3 (a) and (b), three salient points of interest can be designated. First, the insertion of TT increased the friction factor. Second, the larger the TT, the higher the value of f. It is interesting to note that the difference between the stationary and rotating conditions was marginal with the same TT diameter. In fact, the only observable difference was seen with the two lowest Reynolds numbers (Re). 16

Fig. 3. Variation of friction factor with the Reynolds number for TTs under (a) laminar flow and (b) turbulent flow. 3.2 Particle deposition characteristic 3.2.1 Particle deposition velocity We first calculated the particle loss percentage based on more than nine sets of particle concentration measurements at both upstream and downstream locations. The results of the particle loss percentage (LOi) for each bin i are presented in the supplementary material (S2). Here, the particle deposition velocity was computed using equation (6), which used the average particle loss percentage. Figs. 4, 5, and 6 present the particle deposition velocities at various Reynolds numbers for an empty duct and for a duct outfitted with rotating or stationary TTs. As expected, the deposition velocity at a fixed Reynolds number increased as the particle size decreased in all cases. This finding is consistent with many previous studies of the deposition of small particles in the diffusion regime [24,26,44]. The results for the deposition velocity for empty ducts were similar as previous experimental results by Shimada et al. [44], as depicted in Fig. 4. For example, when the Reynolds number (Re) ranged from 5000 to 10,000 (turbulent 17

flow) or from 800 to 1200 (laminar flow), the deposition velocities of group 1 for the empty duct approached 10-3 and 10-4 m/s, respectively.

Fig. 4 Particle deposition velocity (Vd) for the case of an empty duct. As presented in Figs. 4, 5, and 6, a clear increase in the deposition velocity was shown for the turbulent flow region compared to the laminar flow region for an empty duct and all scenarios with TTs, which is consistent with the results reported by Shimada et al. [44]. We believe that this is because the deposition of UFP is affected mainly by the coupling effect of Brownian and turbulent diffusion. For the particle deposition in laminar flow, small particles diffuse to the duct walls as a result of their Brownian motion [46]. Due to the turbulent diffusion involved in turbulent flow, the deposition velocity of UFP under the turbulent condition was consistently higher than that under the laminar condition. Under the turbulent flow, as shown in Figs. 4, 5, and 6, the deposition velocity increased as the Reynolds number increased. This may be a result of more intense turbulent diffusion in airflows with higher Reynolds number. The more intense turbulent diffusion of particles at a higher Reynolds number results in an increased deposition velocity of UFP. For instance, Figs. 5 (b) and 6 (b) depict the variation

18

in particle deposition velocity for a duct with a 10-mm rotating TT. As the Reynolds number increased from 400 to 10,000, the deposition velocity increased to 7.63, 6.62, 4.14, and 5.24 times for particles in groups 1 to 4, respectively. Similarly, for a 10-mm stationary TT, as shown in Figs. 5 (c) and 6 (c), the deposition velocity increased to 5.41, 5.57, 6.55, and 6.88 times, respectively. These increased times of deposition velocity for the 8-mm rotating TT in Figs. 5 (d) and 6 (d) were 6.81, 4.42, 3.80, and 4.78 times, whereas the deposition velocity of the 8mm stationary TT in Figs. 5 (e) and 6 (e) increased to 5.91, 3.78, 4.73, and 5.39 times. It is worth noting that the deposition velocities of UFP by rotating and stationary TTs were obviously higher than that of the empty duct, which suggests that the deposition of UFP was enhanced by the TT inserts. A quantitative comparison between the empty duct and the ducts fitted with TTs is presented as a variation of the particle deposition velocity ratio discussed in the next section.

Fig. 5. Variation of particle deposition velocity (Vd) in laminar flow for the case of an empty duct and TTs: (a) empty duct, (b) rotating 10-mm TT, (c) stationary 10-mm TT, (d) rotating 8-mm TT, and (e)stationary 8-mm TT.

19

Fig. 6. Variation of particle deposition velocity (Vd) in turbulent flow for empty duct and TTs: (a) empty duct, (b) rotating 10-mm TT, (c) stationary 10-mm TT, (d) rotating 8-mm TT, (e) stationary 8-mm TT. 3.2.2 Particle deposition velocity ratio Under consistent pumping power, the deposition velocity ratio (Vd ratio) was calculated for various cases. Figs. 7 and 8 show that the Vd ratios varied with the Reynolds number. The data revealed that the ratios of Vd for rotating TTs and stationary TTs were mostly above unity, which directly validated the UFP deposition enhancement capacity of the TTs in the duct. This can be attributed to the fact that the in-duct TT may induce swirling flows, resulting in stronger turbulent intensity, better fluid mixing in the fluid flow, and a thinner boundary layer (see, for example, [29,42]). Especially for particles in groups 1 and 2, the Vd ratios were higher than in the other two particle groups for most tested Reynolds numbers. The highest Vd ratios in laminar flow and turbulent flow in Figs. 7 and 8 were 6.4 and 7.3 at Reynolds numbers of 400 and 10,000, respectively, both for particle group 1 in the condition of rotating 10-mm TT.

20

In general, Vd ratios, as shown in Figs. 7 and 8, varied from 1.2 to 7.3 for rotating TTs and 0.7 to 4.7 for stationary TTs. Under laminar flow, as shown in Fig. 7, the Vd ratio of the rotating TTs was generally higher than that with stationary TT at the same Reynolds number for all particle size groups. For example, the Vd ratio of the rotating 8-mm TT increased by 7.5%, 61.1%, 123.4%, and 80.2% more than the stationary 8-mm TT at a Reynolds number of 400 for particle groups 1 to 4, respectively in Fig. 7. Under turbulent flow, as depicted in Fig. 8, the Vd ratios of a rotating 8-mm TT at a Reynolds number of 10,000 were 23.9%, 88.2%, 79.7%, and 59.4% higher than those of a stationary 8-mm TT for the four particle groups. Similar results were observed with the rotating and stationary scenarios for the 10-mm TT. This can be explained by the fact that the rotation movement of the TT in the duct directly increases the turbulence of the fluid in the duct and would destroy the laminar boundary layer [41]. In addition to rotational effects, the diameter of the TTs also affected the Vd ratio. This can be verified in Figs. 7 and 8, which show that the highest Vd ratio found in the in-duct 8mm TT under rotation condition was 5.7, whereas that by the 10-mm TT reached 7.3. Regardless of stationary or rotating conditions, an increase in the Vd ratio was seen with the 10mm TT relative to the 8-mm TT. For example, compared with the rotating 8-mm TT, the Vd ratio of the rotating 10-mm TT increased more by 63.3%, 28.1%, 5.2%, and 4.7% at a Reynolds number of 400 for particle groups 1 to 4, respectively. At a Reynolds number of 400, the Vd ratios of the stationary 10-mm TT increased by 30.8%, 60.6%, 45.5%, and 42.7% more than the stationary 8-mm TT for the corresponding four particle groups. At other Reynolds numbers, a higher Vd ratio can be found for the 10 mm TT case. This result was expected and could be due to the increase in the velocity near the duct wall and the reduction effect of the equivalent diameter for the larger TT in the duct airflow [42].

21

Fig. 7. Particle deposition velocity ratio (Vd ratio) of TTs in laminar flows.

Fig. 8. Particle deposition velocity ratio (Vd ratio) of TTs in turbulent flows. 3.3 Performance evaluation on stationary and rotating TTs 22

As discussed in the previous sections, the in-duct TTs inserts can enhance the UFP deposition capacity, and the enhancement of particle deposition is higher for rotating TTs than the stationary TTs. However, they also exhibit an increase in flow resistance compared with an empty duct at the same Reynolds number. Thus, it is necessary to assess the overall particle deposition rate with consideration of the drag penalty. As discussed above, the overall performance index  was established and calculated to achieve a comprehensive evaluation of particle deposition for the ducts with built-in rotating and stationary TTs under laminar and turbulent flows. Figs. 9 and 10 display the performance index of TTs in various flow conditions. Overall, the performance index varied from 1.0 to 6.5 for the rotating TTs and 0.5 to 3.5 and stationary TTs. For the airflow with a given Reynolds number, the overall performance of TTs for particles in group 1 and group 2 was mostly higher than the larger particles within group 3 and group 4 among all cases shown in Figs. 9 and 10. Besides, Fig. 10 shows that all values of the performance index in the turbulent flow were more than unity. Also, the rotating 10-mm TT displayed the best performance among these four TT scenarios under different Reynolds numbers. The highest performance index of rotating 10-mm TT reached 6.5 for particle group 1 at a Reynolds number of 10,000. This is a significant result. Furthermore, as indicated in both Figs. 9 and 10, when comparing the rotating and stationary TT, a similar trend in performance index was obtained. The in-duct rotating TT yielded higher performance than the static TT, indicating that the rotation of TTs could improve the performance of TTs in terms of UFP deposition in the air duct. The significance of this finding is that rotating TTs make a greater contribution to the particle deposition enhancement ratio than the friction factor ratio. This result can be explained by the increase in Vd ratio for the rotating movement of TTs (Figs. 7 and 8). Indeed, a similar 23

friction factor was measured regardless of rotating or stationary TTs (as shown in Fig. 3). Besides, under the same rotating or stationary condition, it can be found that the performance index of the 10-mm TT was generally higher than that of 8-mm TT. It indicates the increase in TT's diameter could increase the performance of in-duct TT. The reason is that the enhancement of UFP deposition velocity by the 10-mm TT was largely higher than that by the 8-mm TT in the duct (as shown in Figs. 7 and 8), although the friction factor increased for the 10-mm TT compared with the 8-mm TT in the duct (as shown in Fig. 3). The variation in performance index lies in the changes in the Vd,TT/Vd,empty (Vd ratio) and the fTT/fempty (friction factor ratio). As shown in Figs. 9 and 10, the TT performance values were higher than 1.0 (indicated by the dashed line) for most scenarios. This means that the effect of TT inserts on the enhancement of deposition velocity was generally larger than that on the increase in friction factor ratio. Such higher enhancement in deposition velocity may be explained by the fact that the insertion of TTs can induce swirl flow and a thinner boundary layer as reported by many studies [29, 31,34]. This induced swirl flow (secondly flow) would result in more intense turbulent diffusion and Brownian diffusion under in-duct TTs conditions compared with that in an empty duct. It is worth noting that the process for particle deposition is complex. The mechanisms for particle deposition may include the diffusion, thermophoresis, inertial impaction, gravitational settling and electrophoresis [18,23,24,26,44]. Effects of inertial impaction and gravitational settling are commonly ignored for UFP deposition [24,26]. Since there is no temperature gradient existing in the flow field and the neutralizer was used in our experiments, the thermophoretic and electrophoresis effects can also be neglected. Thus, Brownian diffusion and turbulent diffusion are considered as the dominant mechanisms for the transport of UFP in the duct flow. 24

Moreover, the rotation could promote the overall particle deposition-hydraulic performance of in-duct TTs as shown in these two figures. It may be due to the enhancing centrifugal force to induce tangential flow (second flow) by the effect of rotation, resulting in promoted mixing of mainstream core and near-wall area [31]. This is consistent with privous works in enhanced heat transfer. In the literature, heat transfer enhancement studies [31,41-42] using rotating TT also reported higher performance than the stationary TT. In addition, it is worth noting that the overall evaluation index (equation (8)) was established for comparing the particle deposition-hydraulic performance of in-duct TTs. It is also applicable for evaluating the particle deposition features of other inserts when the pressure penalty or fanning energy consumption is taken as one of considerations. In our measurements, the same pumping power (fanning power) was employed for testing empty duct and duct fitted with various TTs. As mentiond in the section of data reduction, this criteria has been extensively used in enhanced heat transfer techniques oriented for energysaving [29,39,45]. Besides, in equation (8), m1 is the correlated power of Reynolds number for particle deposition velocity for the empty duct scenario. It also should be noted that the relative trend in particle deposition-hydraulic performance of in-duct TTs would not be affected, even if other values was adopted for m1 since it is common to all inserts.

25

Fig. 9. The performance index of TTs in laminar flows.

Fig. 10. The performance index of TTs in turbulent flows. 4 Conclusions

26

In this work, we proposed the use of in-duct rotating and stationary TTs for enhancement of UFP deposition. The effects of the diameter of TTs and of rotating and stationary TTs on the hydraulic behavior and particle deposition characteristics were experimentally investigated at Reynolds numbers from 300 to 10,000. For each individual measurement, experiments were carried out for an empty duct, 10-mm, and 8-mm TTs for stationary and rotating conditions, all under the same pumping power. A measure of the overall performance index for particle deposition was proposed and separately computed for laminar and turbulent conditions. It is the first study utilizing an active approach (rotating TTs) for particle removal in the ventilation duct and the present results are promising. Flow field measurements or numerical modelling works are important and needed to gain understandings on particle deposition mechanism. Overall, the main conclusions based on the results are listed as follows: (1) The experimental friction factors of the empty duct matched the theoretical models and scaling relations well in both laminar and turbulent flows. We found that TT inserts increase the friction factor over that of an empty duct and that the difference in the friction factor between rotating and stationary TTs was marginal. However, an increase in flow resistance is seen because of the increase in the TTs’ diameter. (2) Turbulent diffusion and Brownian diffusion can both affect the deposition of UFP. Regardless of a scenario of an empty duct or TT duct inserts, the deposition velocity of UFP increases with the Reynolds number and scales inversely with the particle size. In general, in-duct TT inserts are expected to increase the deposition velocity of UFP. That is, the ratios of UFP deposition velocity by TTs to that in the empty duct (Vd ratio) were mostly higher than 1.0. (3) Rotating TTs yielded higher particle deposition velocity ratios than stationary TTs. We found that it varies from 1.2 to 7.3 and 0.7 to 4.7 for rotating and stationary TTs, respectively. In addition, we found that the size of the TTs also affected the Vd ratio and 27

that in-duct TTs with a larger diameter provided a higher Vd ratio of UFP in both laminar and turbulent flows. The highest Vd ratio of UFP with the in-duct 8-mm TT was 5.7 under rotation condition, and that with the 10-mm TT can reach 7.3 for particles less than 20 nm. (4) The performance index of particles less than 20 nm and between 20 to 50 nm featured higher values than that of particles between 50 to 100 nm or between 100 to 150 nm for both rotating and stationary TT conditions. Rotating 10-mm TTs generally offered the best performance index among the four ducts with TT scenarios. The highest performance index of rotating 10-mm TT can reach up to 6.5 for particles smaller than 20 nm. In addition, both the rotation of TTs and an increase in TT's diameter can improve the particle deposition performance in the duct flow.

Acknowledgment This research was fully supported by a grant from Natural Science Foundation of China [project number 51378447], and the General Research Fund of the Research Grants Council of the Hong Kong Special Administrative Region of China [CityU 11210015].

References [1] Vinzents P.S, Møller P, Sørensen M, et al. Personal Exposure to Ultrafine Particles and Oxidative DNA Damage. Environ. Health Perspect., 2005, 113(11):1485-1490. [2] Kecorius S., Madueño L., et al., Respiratory tract deposition of inhaled roadside ultrafine refractory particles in a polluted megacity of South-East Asia. Sci. Total Environ. 2019, 663(1):265-274. [3] Buonanno G, Morawska L, Stabile L. Particle emission factors during cooking activities. Atmos. Environ., 2009, 43(20):3235-3242. [4] Wallace L, Ott W. Personal exposure to ultrafine particles. J Eposure Sci Environ Epidemiol. 2011,21:20-30. [5] Chalupa D C, Morrow P E, Oberdörster G, et al. Ultrafine particle deposition in subjects with asthma. Environ. Health Perspect., 2004, 112(8):879-882. 28

[6] Bhangar S, Mullen NA, Hering SV, Kreisberg NM, Nazaroff WW. Ultrafine particle concentrations and exposures in seven residences in northern California. Indoor Air. 2011, 21:132-144. [7] Stephens B, Siegel J A. Ultrafine particle removal by residential heating, ventilating, and air-conditioning filters. Indoor Air, 2013, 23(6):488-497. Ng B F, Xiong J W, Wan M P. Application of acoustic agglomeration to enhance air filtration efficiency in airconditioning and mechanical ventilation (ACMV) systems. Plos One, 2017, 12(6): e0178851. [8] Lu Z, Lu W Z, Zhang J L, et al. Microorganisms and particles in AHU systems: Measurement and analysis. Building & Environment, 2009, 44(4):694-698. [9] Azimi P., Stephens B. HVAC filtration for controlling infectious airborne disease transmission in indoor environments: Predicting risk reductions and operational costs. Building and Environment, 2013, 70:150-160. [10] Vaughn E., Ramachandran G., Fiberglass vs. synthetic air filtration media, INJFALL, 2002. [11] HVAC HESS - Heating, Ventilation & Air-Conditioning High Efficiency Systems Strategy, Factsheet HVAC Energy Breakdown, 2013. [12] Sippola, Particle deposition in ventilation ducts, University California, Berkeley, 2002. [13] Lai A.C.K., Byrne M.A, Goddard A.J.H., Aerosol deposition in turbulent channel flow on a regular array of three-dimensional roughness elements, J. Aerosol Sci., 2001, 32:121137. [14] Lu H, Lu L, Jiang Y. Numerical simulation of particle deposition in duct air flows with uniform, expanding or contracting cross-section. Energy Build, 2016, 128:867-875. [15] Gao R, Li A. Dust deposition in ventilation and air-conditioning duct bend flows. Energy Convers. Manage., 2012, 55:49-59. [16] Zhan F, Tang J, Ding G, et al. Experimental investigation on particle deposition characteristics of wavy fin-and-tube heat exchangers. Appl. Therm. Eng., 2016, 99:10391047. [17] S.B. Hosseini, R.H. Khoshkhoo, S.M.J. Malabad, Numerical study on poly-disperse particle deposition in a compact heat exchanger, Appl. Therm. Eng., 2017, 127:330–346. [18] Alok Warey, Sandro Balestrino, Pat Szymkowicz & M. R. Malayeri. A One-Dimensional Model for Particulate Deposition and Hydrocarbon Condensation in Exhaust Gas Recirculation Coolers, Aerosol Sci. Technol., 2012,46(2): 198-213. [19] Z. Han, Z. Xu, A. Sun, X. Yu. The deposition characteristics of micron particles in heat exchange pipelines. Appl. Therm. Eng., 2019, 158:113732. [20] M.S. Abd-Elhady, S. Abd-Elhady, C.C.M. Rindt, A.A.v. Steenhoven. Removal of gas-side particulate fouling layers by foreign particles as a function of flow direction. Appl. Therm. Eng., 2009, 29:2335-2343. [21] H. Han, Y.L. He, W.Q. Tao, Y.S. Li, A parameter study of tube bundle heat exchangers for fouling rate reduction, Int. J. Heat Mass Transfer 2014, 72: 210–221. [22] Young, J and Leeming, A. A theory of particle deposition in turbulent pipe flow. J. Fluid Mech., 1997, 340:129-159. [23] Guha, Abhijit. Transport and Deposition of Particles in Turbulent and Laminar Flow. Annual Review of Fluid Mechanics, 2008, 40(1):311-341. [24] Nunayon S S, Zhang H H, Lai A C K. Deposition of ultrafine particle in a duct flow by twisted-tape insert. Building & Environment, 2018, 136:156-161. [25] Lutz Mädler, Friedlander S K. Transport of Nanoparticles in Gases: Overview and Recent Advances. Aerosol Air Qual. Res., 2007, 7:304-342. 29

[26] Hussein T, Smolik, J., Kerminen V M, et al. Modeling Dry Deposition of Aerosol Particles onto Rough Surfaces. Aerosol Sci. Technol., 2012, 46(1):44-59. [27] Zhang H.H., Nunayon S.S., Lai A.C.K. Experimental Study on Deposition Enhancement of Ultrafine Particles in a Duct Flow by Riblets. Appl. Therm. Eng., 2019, 147: 886-894. [28] Liu S, Sakr M. A comprehensive review on passive heat transfer enhancements in pipe exchangers. Renewable Sustainable Energy Rev., 2013, 19(1):64-81. [29] N. Piriyarungrod, Manoj Kumar, C. Thianpong, M. Pimsarn, V. Chuwattanakul, S. Eiamsaard, Intensification of thermo-hydraulic performance in heat exchanger tube inserted with multiple twisted-tapes, Appl. Therm. Eng., 2018, 136:516–530. [30] S. Zhang, L. Lu, C.s. Dong, S. H. Cha. Performance evaluation of a double-pipe heat exchanger fitted with self-rotating twisted tapes. Appl. Therm. Eng., 2019, 158:113770. [31] S. Jaisankar, T.K. Radhakrishnan, K.N. Sheeba, Experimental studies on heat transfer and friction factor characteristics of forced circulation solar water heater system fitted with helical twisted tapes, Sol. Energy 2009, 83: 1943–1952. [32] Esmaeilzadeh E., Almohammadi H., Nokhosteen A., et al. Study on heat transfer and friction factor characteristics of γ-Al2O3/water through circular tube with twisted tape inserts with different thicknesses. Int. J. Therm. Sci., 2014, 82:72-83. [33] M.M.K. Bhuiya, M.S.U. Chowdhury, M. Saha, M.T. Islam, Heat transfer and friction factor characteristics in turbulent flow through a tube fitted with perforated twisted tape inserts, Int. Commun. Heat Mass Transf. 2013, 46: 49–57. [34] Eiamsa-Ard S., Kiatkittipong K. Heat transfer enhancement by multiple twisted tape inserts and TiO2/water nanofluid. Appl. Therm. Eng., 2014, 70(1):896-924. [35] Eiamsa-Ard S., Thianpong C., Eiamsa-Ard P., et al. Convective heat transfer in a circular tube with short-length twisted tape insert. Int. Commun. Heat Mass Transf, 2009, 36(4):365-371. [36] Murugesan P, Mayilsamy K, Suresh S. Turbulent Heat Transfer and Pressure Drop in Tube Fitted with Square-cut Twisted Tape. Chin. J. Chem. Eng., 2010, 18(4):609-617. [37] Jaisankar S, Radhakrishnan T K, Sheeba K N. Experimental studies on heat transfer and thermal performance characteristics of thermosyphon solar water heating system with helical and Left–Right twisted tapes. Energy Convers. Manage., 2011, 52(5):2048-2055. [38] He Y, Liu L, Li P, et al. Experimental study on Heat transfer enhancement characteristics of tube with cross hollow twisted tape inserts. Appl. Therm. Eng., 2018, 131:743-749. [39] Ibrahim, E.Z. Augmentation of laminar flow and heat transfer in flat tubes by means of helical screw-tape inserts. Energy Convers. Manage., 2011, 52:250–257. [40] Cong Qi, Guiqing Wang, Yuying Yan, Siyuan Mei, Tao Luo. Effect of rotating twisted tape on thermo-hydraulic performances of nanofluids in heat-exchanger systems. Energy Convers. Manage., 2018, 166:744-757. [41] Zhang C.C, Wang DB et al., A comparative review of self-rotating and stationary twisted tape inserts in heat exchanger. Renewable Sustainable Energy Rev., 2016, 53:433-449. [42] Munson B.R., Young D.F., Okiishi T.H., Fundamentals of Fluid Mechanics, fifth ed., John Wiley & Sons Inc, 2006. [43] Shimada M, Okuyama K., Asai M., Deposition of submicron aerosol particles in turbulent and transitional flow, AIChE Journal, 1993, 39:17–26. [44] Fan J F, Ding W K, Zhang J F, et al. A performance evaluation plot of enhanced heat transfer techniques oriented for energy-saving. Int. J. Heat Mass Transfer, 2009, 52(12):33-44. 30

[45] Friedlander SK. Smoke, Dust, and Haze. Fundamentals of aerosol dynamics. 2nd ed. Oxford University Press, 2000.

31

Highlights 1. In-duct rotating and static twisted tapes (TTs) could enhance particle deposition. 2. The rotating TTs yielded a higher particle deposition velocity than stationary TTs. 3. A marginal difference in friction factor between static and rotating TTs was seen. 4. A rotation or an increase in diameter of TTs improved their performance. 5. The highest performance index of a rotating TT could reach 6.5.

32