Numerical and experimental investigations of hybrid nanofluids on pulsating heat pipe performance

International Journal of Heat and Mass Transfer 146 (2020) 118887

Contents lists available at ScienceDirect

International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt

Numerical and experimental investigations of hybrid nanofluids on pulsating heat pipe performance M. Zufar a,⇑, P. Gunnasegaran a, H.M. Kumar b, K.C. Ng c a

Institute of Power Engineering, Universiti Tenaga Nasional, Putrajaya Campus, Jalan IKRAM-UNITEN, 43000 Kajang, Malaysia Department of Mechanical Engineering, Sardar Vallabhbhai National Institute of Technology, Surat, Gujarat, India c Department of Mechanical, Materials, and Manufacturing, University of Nottingham Malaysia, Jalan Broga, 43500 Semenyih, Selangor, Malaysia b

a r t i c l e

i n f o

Article history: Received 23 May 2019 Received in revised form 11 October 2019 Accepted 12 October 2019

Keywords: Pulsating heat pipe Start-up pulsations Hybrid nanofluids Thermal resistance Thermal conductivity Viscosity

a b s t r a c t This study investigates the thermal performance of a four-turns Pulsating Heat Pipe (PHP) using a weight concentration of 0.1 wt% Al2O3-CuO hybrid nanofluid, 0.1 wt% SiO2-CuO hybrid nanofluid and water both experimentally and numerically. The start-up pulsations, average evaporator temperatures, thermal resistance, two-phase flow, and non-linear temperature analysis were evaluated with respect to heating power and filling ratio of 10–100 W and 50–60%, respectively. Stability measurement and characterization of thermal conductivity and viscosity properties of hybrid nanofluids were determined. From the experimental results, the thermal resistance SiO2-CuO hybrid nanofluid exhibited was the lowest, i.e. 57% lower than that of water, followed by the Al2O3-CuO hybrid nanofluid, i.e. 34% lower than that of water at the heat input and filling ratio of 80 W and 60%, respectively. Nevertheless, the thermal conductivity and viscosity of Al2O3-CuO hybrid nanofluid were higher than those of SiO2-CuO hybrid nanofluid. The increased viscosity found in Al2O3-CuO hybrid nanofluid would hinder the fluid transportation in PHP, thus augmenting the thermal resistance. Meanwhile, the hybrid nanofluids were able to achieve start-up pulsations earlier and they required lower heating power to reach start-up pulsations as compared to water. At low heating power (below 30 W), the differences in average evaporator temperatures for hybrid nanofluids and water were very small. However, at higher heating power (above 30 W), the differences were significant. The numerical results compared well with those earlier experimental work, thus indicating the reliability of the current numerical simulation. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction Over the past decades, nanofluid has been introduced as a working fluid due to its high thermal conductivity which can significantly improve the thermal performance of a heat transfer system. Nanofluid or mono-nanofluid is a fluid that contains 1–100 nm scale single-type solid particles dispersed in a base fluid [1]. Nanofluid possess great thermal conductivity which makes it very successful in heat transfer as compared to the conventional fluid such as water, ethylene glycol and coolant oil. Nanofluid has been widely used in numerous applications such as medical, thermal processes, and solar energy systems [2–4]. Also, nanofluid has been extensively used in many engineering applications that require cooling. Recently, researchers dispersed two or more types of nanoparticles in a base fluid known as hybrid nanofluid in order to attain the desired rheological characteristics [5]. Hence, hybrid ⇑ Corresponding author. E-mail address: [email protected] (M. Zufar). https://doi.org/10.1016/j.ijheatmasstransfer.2019.118887 0017-9310/Ó 2019 Elsevier Ltd. All rights reserved.

nanofluid can offer unique thermal properties in order to meet the requirement of specific heat transfer application [6–8]. In this modern era, densely packed transistors installed on a microchip would produce high heat flux in an electronic device. Based on Moore’s law, the transistors will be doubled in every two years [9]. The advancements in semiconductor and manufacturing industry have made the placement of millions number of transistors on microchip possible. Therefore, an efficient cooling system is required for heat removal purpose. Heat pipe is a heat transfer device whereby heat is collected and removed at the evaporator and condenser regions, respectively. It is widely used from ground to space application due to its lightweight, reliability and ability to transfer heat efficiently [10]. There are many kinds of heat pipe introduced since past decades, i.e. loop heat pipe, screen mesh heat pipe, micro grooves heat pipe, etc. [11–13]. These heat pipes consist of a wick structure introduced to move the condensate vapor at the condenser region into the evaporator region by capillary action. On the other hand, Pulsating Heat Pipe (PHP) is a unique kind of heat pipe which does not have the wick structure.

2

M. Zufar et al. / International Journal of Heat and Mass Transfer 146 (2020) 118887

Nomenclature A cP Di Do F g h I k K PHP q Q  R RPM S T UV–Vis V

area (m2) centipoise (mPa
s) inner diameter (mm) outer diameter (mm) body force (kg m/s2) gravity acceleration (m/s2) convective heat transfer coefficient (W/m2 K) current (A) thermal conductivity (W/m
K) kelvin (K) pulsating heat pipe heat flux (W/m2) heat input (W) overall thermal resistance (°C/W) revolution per minute source term temperature (°C) Ultra violet-visible spectrophotometer voltage (V)

PHP is a passive two-phase heat transfer device whereby the selfsustenance of fluid flow relies on the pressure imbalance between the condenser and the evaporator. The large pressure difference between the cooling and heating regions generates a pulsated fluid in PHP which is fully thermally driven. This heat transfer mechanism makes PHP a successful heat transfer device due to its affordability and ease of fabrication. However, improving the thermal performance of PHPs is getting more challenging nowadays due to higher heat load demand from the miniaturized electronic industry. Accordingly, nanofluid has been applied in PHP with the intention to cope with the industry demand and to improve the thermal performance of the cooling system. Qu et al. [14] investigated the thermal performance of a sixturns PHP with Al2O3 nanofluid. From the results, the Al2O3 nanofluid showed heat transfer enhancement as compared to water at 50%, 60%, and 70% filling ratio. The thermal resistance of PHP charged with Al2O3 nanofluid was reduced by 32.5% from that of water. Moreover, Riehl et al. [15] conducted an experimental study on PHP using copper nanofluid as the working fluid. They reported that by adding copper nanoparticles in water, the number of nucleation sites would increase which was beneficial for bubble formation. Greater pulsation can be generated with more bubble formation, thus improving the overall thermal performance of PHP. Tanshen et al. [16] studied the thermal performance of MultiWalled Carbon Nanotube (MWCNT) nanofluid in PHP. The study was conducted at 60% optimal filling ratio. Four weight concentrations, i.e. 0.05 wt%, 0.1 wt%, 0.2 wt%, and 0.3 wt% were tested in a six-turns PHP. It was reported that the thermal performance of PHP was related to the temperature fluctuation or pulsations over time. All MWCNT nanofluids showed greater pulsations which could significantly enhance the heat transfer rate of PHP. The 0.1 wt% MWCNT nanofluids exhibited the greatest fluctuation frequency with respect to time and meanwhile, it gave the lowest thermal resistance as compared to nanofluids with other weight concentrations and water. Nazari et al. [17] investigated the heat transfer enhancement of PHP charged with graphene oxide nanofluid. The thermal resistance was reduced by 42% (as compared to water) when PHP was filled with graphene nanofluid at 0.25 wt%. However, at higher nanofluid concentration, the thermal performance deteriorated due to larger dynamic viscosity. In addition, Xing et al. [18] reported better start-up characteristic and significant improvement on the thermal performance of PHP when

VOF

volume of fluid

Greek characters a thermal diffusivity (m2/s) b expansion coefficient (1/K) m dynamic viscosity (kg/ms) m kinematic viscosity (m2/s) q density of fluid (kg/m3) u particle weight concentration Subscripts bf base fluid c condenser e evaporator nf nanofluid p nanoparticle s solid

the weight concentration of MWNTs nanofluid was below 0.3 wt %. The authors reported that PHP incorporated with 0.1 wt% MWNTs nanofluid showed the lowest thermal resistance as compared to nanofluids with other weight concentrations of MWNTs. MWNTs nanofluid at larger weight concentration is more viscous. This is unfavorable because the start-up time can be prolonged and hence MWNTs nanofluid at larger weight concentration is not recommended for heat transfer enhancement. Based on above literature, the usage of mono-nanofluid in PHP to improve heat transfer performance has been broadly investigated and developed for past decades [12–20]. Nevertheless, the number of research works related to the use of hybrid nanofluid in PHP is quite limited. Therefore, the present study attempts to fill this gap by performing experimental and numerical investigations on the thermal performance of PHP charged with 0.1 wt% Al2O3CuO hybrid nanofluid and 0.1 wt% SiO2-CuO hybrid nanofluid under various heat inputs and filling ratios. The thermal performances are then compared with that of water. Previous studies have shown that the weight concentration would affect the overall heat transfer enhancement of PHP. Higher weight concentration would worsen the thermal performance of PHP due to higher fluid resistance. Various researchers found that the highest thermal performance could be attained when dispersing 0.1% weight concentration of nanoparticles in the base fluid. In the current work, the Al2O3-CuO and SiO2-CuO hybrid nanofluids used in PHP were prepared using 0.1% weight concentration. The present study focuses on the preparation methods, the characterizations of thermal conductivity and viscosity evaluated at various temperature ranges, the stability analysis of the hybrid nanofluids and its influence on the start-up pulsations, the average evaporator temperature, and thermal resistance of PHP under various heat inputs and filling ratios. Also, the numerical results on the temperature behavior, the two-phase flow phenomena and the non-linear temperature variation of PHP charged with hybrid nanofluids were analyzed. 2. Materials and method 2.1. Experimental setup and procedure The schematic diagram of the experimental setup for PHP is shown in Fig. 1. It comprised a vacuum pump, a PHP, a data logger,

M. Zufar et al. / International Journal of Heat and Mass Transfer 146 (2020) 118887

3

Fig. 1. Schematic diagram of PHP experimental setup.

a control panel and a power supply. A four-turns PHP made from copper was enclosed with an aluminum block of dimension 150 mm  50 mm  25 mm at the evaporator section. Five circular cartridge heaters with diameter and length of 6 mm and 40 mm respectively were installed inside the aluminum block. The heaters were alternately placed between the four-turns copper pipe in the aluminum block in order to provide sufficient and even heating. A steel block was used to cover the adiabatic section. Glass wool was densely packed in the steel block to provide thermal insulation. The condenser section consisted of three pipe turns and a 50 mm long copper tube was exposed to room temperature for heat removal purpose. Dimmer stat was used to manipulate the heat supply to the evaporator by varying the voltage and current between 0–60 V and 0–5 A respectively to the cartridge heaters. Digital ammeter (model: MA 12) and voltmeter (model: MV 15) were used to measure the current and voltage at 0:5% accuracy. An oil rotary vacuum pump (Model VS 114 DT, HINGHIGH) was used to create a vacuum up to 0.01 mbar in the PHP. Fig. 2(a) shows the PHP test rig used to conduct the experimental investigations in the present study. The orientation bracket acted as structural support, where PHP was attached to the structure in order to hold the PHP vertically. Copper was used as PHP material because copper has high thermal conductivity. Likewise, the evaporator section was encapsulated with aluminum block in order to distribute the heat supplied by the heating coils evenly. On the contrary, a steel block (a poor heat conductor) was used to house the adiabatic section in order to minimize heat loss. The charging and discharging valves were installed at the top left and right corners of the condenser section in order to fill and evacuate the working fluid into and from the PHP, respectively. Fig. 2(b) illustrates the evaporator, adiabatic and condenser sections of lengths 50 mm, 100 mm, and 50 mm respectively with a gap of

20 mm between the turns. The wall temperatures at different sections were measured by seven K-type thermocouples of accuracy  ±0:5 C as shown in Fig. 2(b). The thermocouples at the adiabatic section were attached at the outer part of the steel block in order to detect any significant heat loss from this region. The outer and inner diameters of the PHP are 3 mm and 2 mm respectively as shown in Fig. 2(c). Mameli et al. [21] and Qu et al. [14] investigated the effect of filling ratio at various heat inputs. The optimal filling ratios were reported as 50% and 60% hence, these filling ratios were selected in the present study. The ENVADA data scanner was used to scan and send the temperature data from the thermocouples to the computer via a USB cable. Initially, the vacuum pump was operated for 15 min to evacuate the excess working fluid and to create a vacuum pressure in PHP. Then, the working fluid of filling ratio 50% from the total volume of PHP was drawn into a syringe and injected into the PHP through the charging valve. All equipment described earlier were turned on. The thermocouples readings were observed for about 5–10 min to ensure the readings were similar to the room temperature. Next, the heating power was adjusted to the desired heat input. The heating power was increased once the system temperature was steady. The experimental work was conducted using water as the working fluid initially, followed by Al2O3-CuO hybrid nanofluid and SiO2-CuO hybrid nanofluid. Similar experimental procedures were conducted for PHP at 60% filling ratio. 2.2. Numerical model and governing equations In the present numerical study, a three-dimensional (3D) fourturns PHP simulation model was generated using ANSYS FLUENT. The PHP geometry was divided into three sections, i.e. evaporator,

4

M. Zufar et al. / International Journal of Heat and Mass Transfer 146 (2020) 118887

Fig. 2. (a) Test rig (b) dimensions and (c) cross section diameter of PHP.

adiabatic and condenser. Hybrid nanofluids and water were used as the working fluid in the PHP. The thermal conductivity and the dynamic viscosity of hybrid nanofluids and water were obtained experimentally in the present study. The k-epsilon model was adapted for turbulence closure. The Lee’s model was utilized to simulate the evaporation and condensation phenomena in the PHP. PISO model was used for the pressure-velocity coupling due to suitability for transient flow problem. The time step size of 104 s was employed for all cases. Hexahedral meshes were configured throughout the domain for better accuracy. Grid Independence Test (GIT) was conducted in order to determine the suitable grid size for the current numerical model. The average evaporator temperatures simulated using three different grids sizes for PHP charged with water at 60% filling ratio at heat input of 30–100 W is reported in Fig. 3. As seen, the result obtained using the grid size of 0.04 mm was the closest to the experimental one. Hence, the grid size of 0.04 mm was used in the present numerical study. Volume of Fluid (VOF) model was incorporated in this numerical study since PHP is a thermal system consisting of two-phase liquid-vapor flow. VOF model is selected due its ability to trace the interfacial position of two or more immiscible fluids. Under VOF, all fluids share a single set of momentum equations and the volume fraction in each computational cell is tracked throughout the domain. In the cell, the qth fluid’s volume fraction is regarded as aq. Referring to Hsu et al. [22], the qth phase can be computed from the following:

1

qq



 
 Xn   @
aq qq þ r: aq qq v ¼ Saq þ p¼1 m_ pq  m_ qp @t

ð1Þ

whereby the mass transfer from phase q to phase p is denoted as _ pq is the mass transfer of phase p to phase q. The _ qp . Likewise, m m Saq is the source term and it was set to zero in the current work. As a single momentum equation is used, the resulting velocity field

Fig. 3. GIT of the average evaporator temperatures of PHP charged with water at 60% filling ratio under heat input ranging from 30 W to 100 W.

is shared among the phases. The momentum equation can be written as

   @
! q v þ r:ðqvv Þ ¼ rp þ r: l rv þ rv T þ qg þ F @t

ð2Þ

Surface tension and contact angles in PHP were considered in the current work as their effects on the fluid flow are quite apparent [23–25]. The surface tension force can be written as the volumetric force [22]:

F v ol ¼ r12 1 2

qk1 ra2 ðq1 þ q2 Þ

ð3Þ

M. Zufar et al. / International Journal of Heat and Mass Transfer 146 (2020) 118887

The thermo-physical properties of the nanofluids are dependent on those of the component phases. In the vapor-liquid two-phase system, the thermal conductivity, the specific heat, the density and the viscosity can be expressed as [26–29]:

knf ¼ 

qC p

  kp þ 2kbf þ 2 kp  kbf /   kp þ 2kbf  kp  kbf /

 nf

    ¼ ð1  /Þ qC p bf þ / qC p p

ð4Þ ð5Þ

qnf ¼ ð1  /Þqbf þ /qp

ð6Þ

lnf ¼ lbf ð1 þ 2:5/Þ

ð7Þ

Here, u is the particle weight concentration. The subscripts nf, bf and p denote nanofluid, base fluid, and nanoparticle, respectively. Energy equation was activated in the present numerical study [30]:

  @ ðqEÞ þ r:ðv ðqE þ pÞÞ ¼ r: keff rT þ Sh @t

ð8Þ

The energy E, and the temperature T can be computed in the following manner [31]:

Pn

a qq Eq q¼1 aq qq

q¼1 q

E ¼ Pn

ð9Þ

where Eq is calculated based on the specific heat of q phase and the shared temperature. The source term, Sh was set to zero and the properties q and keff (effective thermal conductivity) were averaged from all phases. At the evaporator section, the mathematical form of the boundary condition is shown in Eq. (10). The constant temperature boundary condition (fixed as T) was employed. Te is the average evaporator temperature obtained experimentally based on the heat input. The heat flux supplied and the convective heat transfer coefficient at the evaporator region are denoted as q and he, respectively.

T¼

q þ Te he

ð10Þ

%weight
concentration ¼

wnp  100 wbf þ wnp

5

ð13Þ

The preparation of 500 ml of 0.1 wt% SiO2-CuO hybrid nanofluids was initially conducted by adding about 0.25 g of SiO2 nanoparticles into 249.75 ml (249.75 g) of water in order to obtain 0.1 wt% SiO2 mono-nanofluid. Similarly, about 0.25 g of CuO nanoparticles were mixed in the same volume of water to produce 0.1 wt% CuO mono-nanofluid. Next, the SiO2 mono-nanofluid and CuO mononanofluid were magnetic-stirred separately for 15 min without any heat addition. Then, the SiO2 mono-nanofluid and the CuO mono-nanofluid were added together and the mixture was subjected to magnetic stirring for 30 min to obtain the 0.1 wt% SiO2CuO hybrid nanofluid. Further reduction of agglomeration and clustering of the SiO2CuO hybrid nanoparticles in the base fluid water was achieved using two different methods, i.e. high shear mixing method (Ultra-speed homogenizer) and ultra-sonication method (Ultrasonic bath). The ultra-speed homogenizer utilizes a rotational shearing force of 10,000 RPM, whereas the ultrasonic bath enforces heating power of 80 W at frequency of 37 Hz. The SiO2-CuO hybrid nanofluid was placed under the ultra-speed homogenizer and ultrasonic bath at different durations, i.e. 30 min, 40 min, 50 min and 120 min, 240 min and 360 min, in order to produce six samples of SiO2-CuO hybrid nanofluids. The reason that the SiO2-CuO hybrid nanofluid was prepared using ultra-speed homogenizer and ultrasonic bath sonication was to analyze the stability and to identify the best preparation method and duration for preparing the hybrid nanofluids in the present study. Ultra Violet-Visible spectrophotometer (UV–Vis) was used to conduct the stability analysis for the six SiO2-CuO hybrid nanofluid samples. The evaluation of stability hybrid nanofluid using UV–Vis has been considered by Nabil et al. [34], Hamid et al. [35] and Che Sidik et al. [36]. It is commonly known that colloidal stability would greatly affect the thermo-physical properties of hybrid nanofluid. Hence, the present study utilized UV–Vis to experimentally determine the stability of SiO2-CuO hybrid nanoparticles in water prepared using the ultra-speed homogenizer and the ultrasonic bath at different durations. Fig. 4 shows the experimental results of the stability analysis. The absorbance ratios of the ultra-speed homogenizer and the

The condenser section was assumed to undergo natural convection [32]:

q ¼ hAc ðDTÞ

ð11Þ

where h, Ac and DT are convective heat transfer coefficient, surface area of condenser region, and temperature difference between the fluid and the condenser, respectively. On the other hand, zero heat flux boundary condition was imposed at the adiabatic section [33]:

q ¼ ks

@T s @n

ð12Þ

where ks is the thermal conductivity of copper (385 W/m
K), Ts is the temperature of solid surface at the adiabatic region and n is the direction normal to the wall. 2.3. Preparation, stability and characterization of hybrid nanofluid In the present study, three types of nanoparticles, i.e. SiO2, CuO, and Al2O3 were dispersed in the base fluid (i.e. water) to form SiO2CuO and Al2O3-CuO hybrid nanofluids. The two-step method was utilized to prepare the hybrid nanofluids with (50:50) nanoparticles mixing ratio. Eq. (13) was used to calculate the weight concentration of 0.1 wt% hybrid nanofluids in the present study [33], whereby the weights of nanoparticles and base fluid were denoted as Wnp and Wbf, respectively.

Fig. 4. Stability analysis of UV–Vis. Absorbance ratio against sedimentation time obtained using different preparation methods of SiO2-CuO hybrid nanoparticles are plotted.

6

M. Zufar et al. / International Journal of Heat and Mass Transfer 146 (2020) 118887

ultrasonic bath at different durations against the time taken for the SiO2-CuO hybrid nanoparticles to sediment in the base fluid water were plotted. The absorbance ratio of 100% depicts the highest colloidal stability of hybrid nanoparticles over a period of time. In general, the stability is dependent on both the preparation method and duration. Based on Fig. 4, the ultrasonic bath with 360 min sonication produced the most stable sample, as the absorbance ratio of more than 0.8 (80%) up to 200 h of sedimentation time was attained. Meanwhile, all the SiO2-CuO hybrid nanofluid samples that were prepared using the ultra-speed homogenizer showed absorbance ratio of lower than 0.8 in less than 50 h sedimentation time. In addition, the SiO2-CuO hybrid nanofluid samples prepared using ultra-speed homogenizer exhibited significant deterioration in absorbance ratio or stability of hybrid nanoparticle dispersion in base fluid as compared to those prepared using ultrasonic bath. From Fig. 4, longer duration of preparation would lead to better stability of SiO2-CuO hybrid nanoparticles. This indicates that the sonication time of 360 min should be used. Therefore, the 0.1 wt% Al2O3-CuO hybrid nanofluid and the 0.1 wt% SiO2-CuO hybrid nanofluid in the present study were prepared using the ultrasonic bath method with 360 min sonication time in order to achieve the most stable colloidal hybrid nanoparticles in water. The characterizations of thermal conductivity and viscosity of the hybrid nanofluids and water in the present study were conducted experimentally. The thermal conductivity was measured using KD2-Pro Thermal Property Analyzer. A (KS-1) needle sensor was used in the measurement. The glycerin sample provided by the supplier was used for verification purposes. The given sample’s thermal conductivity was within 3.5% of the measured thermal conductivity (i.e. 0.286 W/m
K) at 25 °C. Samples of hybrid nanofluids were immersed in a water bath to regulate the temperature. The thermal conductivity of hybrid nanofluids were then measured ranged from 50 °C to 80 °C. The average thermal conductivity value was then computed from four temperature readings measured for each hybrid nanofluid. Brookfield LVDV Ultra Rheometer was used to determine the viscosity of hybrid nanofluid. The data was measured within the torque range of 10– 100% and the average value was then obtained from the three repeated datasets. Error analysis was conducted and the error was found to be within 5% when water at temperature ranging from 50 °C to 70 °C was used with reference to Selim et al. [8] results. The FEI Tecnai F20 XT field emission transmission electron microscope was used to capture the Transmission Electronic

Microscope (TEM) images of Al2O3-CuO and SiO2-CuO hybrid nanofluids as shown in Fig. 5(a) and (b). From the high magnification images, two different kinds of nanoparticles were well suspended in the water, forming the desired hybrid nanofluid in the current study. 2.4. Data reduction and uncertainties Experimental studies are inevitable from any error, thus consideration of data uncertainty is important. Data reduction from all the design parameters tested in the present study were calculated using the following equations: 

R ¼ ðT e  T c Þ=Q  Q loss

ð14Þ

T e ¼ ðT 1 þ T 2 þ T 3 Þ=3

ð15Þ

T c ¼ ðT 6 þ T 7 Þ=2

ð16Þ

Propagation method was used to analyze the uncertainties of the measured design parameters [18]. The thermal resistance of 

PHP is denoted by R. Te is the average evaporator temperature of PHP. Thermocouples T1, T2, and T3 were positioned at the evaporator section as shown in Fig. 2(b) to probe temperature of the region. Likewise, Tc is the average condenser temperature, whereby T6 and T7 were located at the condenser section of PHP for temperature measurement. Negligible heat loss, Qloss was measured at the outer aluminum block of the evaporator section, hence the heat loss was neglected. Heat input, Q can be calculated by following equation:

Q ¼V I

ð17Þ

where V and I are the voltage and current of the heating system respectively. In the present work, the heat input power calculated by the current and voltage having an accuracy of 0.5%. The voltage and current have ranged between 0–60 V and 0–5 A, respectively. For heat input power of 30 W, the minimum current and voltage were 3.2 A and 10.2 V, respectively. Hence, the uncertainty of the heat input power can be given as follows:

dQ ¼ Q

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi  2  2  2  2 dV dI 60  0:5 5  0:5 ¼ þ þ V I 10:2 3:2

¼ 3:04%

Fig. 5. Transmission Electronic Microscope (TEM) images of (a) Al2O3-CuO and (b) SiO2-CuO hybrid nanofluids.

ð18Þ

7

M. Zufar et al. / International Journal of Heat and Mass Transfer 146 (2020) 118887

The uncertainty of 30 W heat input calculated was 3.04%. The maximum uncertainty of heat input power calculated was 3.97% for heat input power of 10 W and the value was seen to reduce as the heat input was increased. The minimum uncertainty calculated was 1.72% for heat input power of 90 W. The uncertainty of the evaporation and condensation temperature was given by 0.5 °C. The following equation was utilized to calculate the uncertainty on thermal resistance:

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi  2  2  2 dT e dT c dQ þ þ  ¼ Q T  T T  T e c e c R qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ¼ ð0:5Þ2 þ ð0:5Þ2 þ ð3:04Þ2 ¼ 3:12% 

dR

ð19Þ

The thermal resistance uncertainty calculated was obtained with a value of 3.12%. Thus, the uncertainty analysis on heat input power and thermal resistance were between 1.7 and 4% which is within an acceptable ranged. 3. Results and discussion 3.1. Thermal conductivity and viscosity of hybrid nanofluids Table 1 depicts the experimental results on the characterization of the thermal conductivity and viscosity of Al2O3-CuO hybrid nanofluid, SiO2-CuO hybrid nanofluid and water at temperatures ranged of 50–80 °C. Based on Fig. 6(a), the thermal conductivity of hybrid nanofluids and water increases with an increase in temperature. This indicates that the increment of temperature contributes to the increase of the thermal conductivity of the hybrid nanofluids. Thermal conductivity of Al2O3-CuO hybrid nanofluid and SiO2-CuO hybrid nanofluid were seen to enhance by up to 15% and 10%, respectively compared to water at 80 °C. This shows that suspension of hybrid nanoparticles have increased the thermal conductivity of water due to its solid nanoparticles which are high in thermal conductivity [37]. High thermal conductivity value will have ability to transfer heat at much greater rate and eventually improves the thermal performance of PHP. Alternatively, referring to Fig. 6(b), the viscosity of the hybrid nanofluids and water have an inversely proportional relationship against temperature. The viscosity of Al2O3-CuO hybrid nanofluid and SiO2CuO hybrid nanofluid at 80 °C were augmented by 37% and 26%, respectively compared to water. Large value of viscosity will resist the fluid flow in PHP which is undesirable for an efficient heat transfer, whereby lower viscosity is favorable to enhance the thermal performance of PHP. 3.2. Thermal performance of PHP 3.2.1. Start-up pulsations of PHP In the aforementioned text, the thermocouples at the adiabatic region were attached at the outer part of the steel block of the PHP which function to detect heat losses from this region. The temperature readings represent were approximately at room temperature, which indicates no excessive heat loss of the adiabatic section. In the present study, the heat input tested was ranged between

Fig. 6. (a) Thermal conductivity and (b) viscosity of hybrid nanofluids and water at temperature ranging from 50 °C to 80 °C.

10 W and 100 W. Heat input was gradually increased in a step of 10 W before reached 50 W since start-up mechanism is crucial and occur at low heat input range as reported by Riehl et al. [15] and Patel et al. [38]. The increment of heat input was set in step of 20 W after heat supply to the evaporator section reached 50 W. Start-up mechanism is defined by the onset pulsations of the two-phase liquid vapor slugs in PHP. When heat was supplied to the evaporator section, the temperature in the region increased which caused vaporization of the working fluid. Bubbles presence in the evaporator section coalescence among each other to form vapor slugs in PHP as the heat supply was increased. On the other

Table 1 Measured thermal conductivity and viscosity of hybrid nanofluids and water at temperature ranging from 50 °C to 80 °C. Temperature (°C)

50 60 70 80

Al2O3-CuO/water

SiO2-CuO/water

Water

Thermal conductivity (W/m
K)

Viscosity (cP)

Thermal conductivity (W/m
K)

Viscosity (cP)

Thermal conductivity (W/m
K)

Viscosity (cP)

0.76 0.78 0.80 0.81

0.69 0.65 0.62 0.58

0.70 0.72 0.74 0.76

0.64 0.60 0.55 0.49

0.64 0.66 0.68 0.69

0.59 0.45 0.38 0.36

8

M. Zufar et al. / International Journal of Heat and Mass Transfer 146 (2020) 118887

hand, at the condenser section which was lower in temperature creates an imbalance of pressure in PHP which forces the high pressure vaporized fluid to move into the condenser region which was lower in pressure. At this stage, the formation of liquid vapor slugs can be observed and pulsations have started in PHP. The liquid vapor slugs at the condenser region dissipated heat, whereby the vaporized fluid condensed into liquid and returned to the evaporator section to absorb heat again. This depicts the complete working mechanism of PHP. The success operation of PHP depends on the two-phase liquid vapor slug oscillations which are thermally driven. There are various parameters that influence the thermal performance of PHP such as the number of turns, heat input, tube diameter, filling ratio, orientation and thermal physical properties of working fluid. In the present study, the results are discussed based on the filling ratio, heat input and thermal physical properties of working fluid on the thermal performance of PHP charged with hybrid nanofluids and to compare with water. Fig. 7 illustrates the start-up pulsations of PHP for hybrid nanofluids and water at 50% filling ratio. At heat input of 10 W, the temperatures for all the working fluid tested was seen increasing, accordingly it raised the wall temperature of the evaporator and condenser section. It was observed at 10 W heat input, the evaporator section was not sufficient to start the pulsations for the hybrid nanofluids and water since there was no fluctuation occurred in the evaporator and condenser temperatures, thus indicates it as a free pulsation region. With increment of heat input at 20 W, start-up pulsation was shown for Al2O3-CuO hybrid nanofluid and SiO2-CuO hybrid nanofluid. This point out the passive heat transfer mechanism in PHP had been initiated. Differently for water, the onset pulsation started at much later time when the heat input was supplied at 30 W. This was due to the high thermal capability of hybrid nanofluid which able to collect heat at much greater rate. Referring to Table 1, the hybrid nanofluids have greater thermal conductivity than water which allows for proficient heat absorption which leads to faster pulsations in PHP. However, the pulsations were seen greater when using water in PHP as compared to the hybrid nanofluids. This was because water possesses lower viscosity than the hybrid nanofluids, hence this reflects on the fluctuations of the temperatures and oscillates more vigorously in PHP. The results shown in Fig. 8 represents the start-up pulsations of Al2O3-CuO hybrid nanofluid, SiO2-CuO hybrid nanofluid, and water at filling ratio of 60% in PHP. It was observed that the Al2O3-CuO hybrid nanofluid and SiO2-CuO hybrid nanofluid have started pulsations at 20 W heat input. When using water as a working fluid in PHP, the pulsations were seen to occur at heating power of 40 W. The onset pulsations of water at 60% filling ratio was longer and requires more heat to be supplied in order to start the pulsations. However, the start-up time for the hybrid nanofluids was not affected by increment of the filling ratio. This was due to the high thermal conductivity of these respective hybrid nanofluids which capable of absorbing heat at larger capacity in much quicker time. The results indicated that hybrid nanoparticles suspension in water possess better heat transfer capability, thus reduced the time and heat input required to start the pulsations in PHP. Whereas, PHP charged with water had prolonged the start-up time and requires greater heat input to start the pulsations. Moreover, it was shown that the temperatures difference between the evaporator and condenser region was larger for water, while the hybrid nanofluids were much smaller at both filling ratio of 50% and 60%. Goshayeshi et al. [39] reported similar case whereby the addition of nanoparticles in the base fluid water have reduced the temperatures difference between the evaporator and condenser. Also observed from Fig. 7 and Fig. 8, PHP with 60% filling ratio exhibited smaller temperature difference between the evaporator and condenser region relative to the PHP with 50% filling ratio.

Fig. 7. Start-up pulsations of hybrid nanofluids and water in the PHP at 50% filling ratio.

3.2.2. Average evaporator temperatures of PHP Fig. 9(a) and (b) represent the average evaporator temperature (Te) under heat input of 10–100 W for Al2O3-CuO hybrid nanofluid, SiO2-CuO hybrid nanofluid, and water at PHP filling ratio of 50% and 60%, respectively. It was observed that at heat input of 10– 20 W, the Te showed insignificant difference for all the working fluid. However, with subsequent heating above 30 W, the Te for the hybrid nanofluids were lower analogous to water. It was observed, Al2O3-CuO hybrid nanofluid and SiO2-CuO hybrid nanofluid exhibited reduction in the Te by 28 °C and 36 °C, respectively

M. Zufar et al. / International Journal of Heat and Mass Transfer 146 (2020) 118887

9

Fig. 9. Average temperatures of evaporator (Te) of PHP charged with hybrid nanofluids and water at (a) 50% and (b) 60% filling ratio at various heat inputs.

Fig. 8. Start-up pulsations of hybrid nanofluids and water in the PHP at 60% filling ratio.

as compared to water under heating power of 80 W. This was due to the high thermal conductivity of these hybrid nanofluids which enable them to absorb and dissipate heat more efficiently at the evaporator and condenser section, respectively. The Te exhibited by hybrid nanofluids were lower under all the heat input tested compared to water, owing to the high heat transfer rate of the hybrid nanofluids. Moreover, the SiO2-CuO hybrid nanofluid displayed lower Te in comparison to the Al2O3-CuO hybrid nanofluid. This can be explained due to the effect of high viscosity that hinders the fluid flow in PHP when incorporating Al2O3-CuO hybrid nanofluid as a working fluid. As a result of this high shear resistance in PHP, the heat carried by the fluid was transferred ineffi-

ciently and this result agrees with what had been reported by Xing et al. [18]. The SiO2-CuO hybrid nanofluid possesses lower viscosity, therefore it was able to circulate more smoothly in PHP and transfer heat much efficiently, consequently contributes to the lower Te. It was observed in Fig. 9(a) and (b) that the 60% filling ratio PHP charged with water at 100 W heating power obtained lower Te by up to 20 °C in regards to the PHP charged with water at 50% filling ratio. Likewise, the hybrid nanofluids with 60% filling ratio of PHP were able to maintain lower Te under heat input above 30 W as compared to the PHP with 50% filling ratio. However, at heat input below 30 W the Te for all the working fluid does not have significant differences between the 50% and 60% PHP filling ratio. PHP charged with SiO2-CuO hybrid nanofluid at 60% filling ratio under 100 W heat input showed decrease of Te by 27 °C corresponding to its identical in the 50% filling ratio. This was because, at high heating power, the sensible heat carried by the liquid and latent heat transport of vapor were greater for PHP with 60% filling ratio, thus efficiently promotes the passive oscillation in PHP which gives higher heat transfer rate. However, at 50% filling ratio of PHP, the sensible and latent heat transported was not substantial, thereupon yielded much higher Te.

10

M. Zufar et al. / International Journal of Heat and Mass Transfer 146 (2020) 118887

3.2.3. Thermal resistance of PHP Thermal resistance was measured in order to determine the thermal performance of PHP charged with Al2O3-CuO hybrid nanofluid, SiO2-CuO hybrid nanofluid, and water. Fig. 10 depicts the present experimental results of thermal resistance of PHP incorporated with hybrid nanofluids and water at both 50% and 60% filling ratio and compared with experimental results reported by various researchers from the literature. It was exhibited that the thermal resistance was decreased, as the heat input was increased. This was due to the two-phase heat transfer mechanism in PHP which can be achieved at higher heat input. Thus, the thermal resistance can be significantly reduced at higher heat input. The Al2O3-CuO hybrid nanofluid and SiO2-CuO hybrid nanofluid at both filling ratio of 50% and 60% yielded lower thermal resistance at all heat input tested as compared to the water. The improvement in thermal performance of the hybrid nanofluids were mainly due to the fact of numerous creations of nucleation sites caused by the hybrid nanoparticles dispersion in water which have greatly promoted growth of bubbles in PHP. Larger heat can be absorbed and dissipated when more vapor slugs were formed in the PHP. Furthermore, the presence of hybrid nanoparticles intensified the displacement and generation of vapor slugs in the base fluid, hence boost the pulsating motion in PHP. As a result, greater heat energy can be carried by these vapor slugs and more heat removal was taken place at the condenser region. Accordingly, the temperature difference between the evaporator and condenser region was reduced and therefore enhanced the thermal resistance of PHP. The PHP charged with SiO2-CuO hybrid nanofluid and Al2O3-CuO hybrid nanofluid at 60% filling ratio under 80 W heat input exhibited thermal resistance enhancement by up to 57% and 34%, respectively in comparison to water. The hybrid nanoparticles added led to the augmentation of the thermal conductivity of the base fluid water, hence contributed to the enhancement of the thermal resistance of PHP. The Al2O3-CuO hybrid nanofluid possesses higher thermal conductivity in comparison to SiO2-CuO hybrid nanofluid and water at all temperatures ranged as shown in Table 1. However, due to larger viscosity of Al2O3-CuO hybrid nanofluid, it had tarnished the thermal performance of PHP, thereupon caused slight increase in the thermal resistance value in comparison to the PHP charged with SiO2CuO hybrid nanofluid. The fluids were unable to move effortlessly

from the evaporator section to the condenser region due to high shear resistance in the PHP wall, subsequently more heat cannot be transferred and dissipated off the condenser. This has caused large accumulation of heat at the evaporator section which had increased the temperature difference between the evaporator and condenser region, accordingly led to the increment of the thermal resistance. Nazari et al. [40] reported similar cases whereby the nanofluid had worsened the thermal performance of PHP due to high viscosity, despite high thermal conductivity of the nanofluid. As a deduction, the SiO2-CuO hybrid nanofluid was better in thermal performance compared to Al2O3-CuO hybrid nanofluid at all filling ratio tested. On the other hand, it was observed PHP charged with hybrid nanofluids and water at 60% filling ratio obtained better thermal resistance compared to the 50% filling ratio. This result agrees with previous researchers who obtained best thermal performance of PHP charged with nanofluids at an optimal filling ratio of 60% in their experimental studies [15,40– 41]. On the other hand, based on Fig. 10, the present experimental results of PHP charged with hybrid nanofluids and water were compared with experimental results by Tanshen et al. [16], Jia et al. [42] and Wu et al. [43] who conducted studies on PHP filled with mono-nanofluids. It was observed that the PHP charged with hybrid nanofluids in the present study obtained lower thermal resistance than the PHP charged with mono-nanofluids. This clarifies the potential of hybrid nanofluid than mono-nanofluid and elucidates the efficacy of incorporating hybrid nanofluid as a working fluid to achieve better thermal performance of PHP and other heat transfer systems.

Fig. 10. Comparison of the present measured thermal resistances for PHP charged with hybrid nanofluids and water at 50% and 60% filling ratio against other researcher’s experimental results.

Fig. 11. Comparison of the present simulated and measured average evaporator temperatures of PHP against the experimental results of Viput et al. [38]. 50% filling ratio was used.

3.3. Numerical results The present transient simulation was carried out on PHP charged with hybrid nanofluids and water at filling ratio of 50% and 60% under various heat input to analyze the temperature contour, two-phase flow behavior, and non-linear temperatures analysis. 3.3.1. Numerical validation and temperature contour Fig. 11 demonstrates a comparison of the present numerical results with the present and Vipul et al. [38] experimental results

M. Zufar et al. / International Journal of Heat and Mass Transfer 146 (2020) 118887

for average evaporator temperatures of PHP filled with water at 50% filling ratio under various heat input. The deviation of the present numerical with the present and Vipul et al. experimental results were found to be within 8.2% which is an acceptable range. Fig. 12(a)–(c) illustrate the temperature contour of cross-section of PHP filled with Al2O3-CuO hybrid nanofluid, SiO2-CuO hybrid nanofluid, and water at 3 s time, under heating power of 30 W at 50% filling ratio. The fluid and wall temperature of PHP were seen to be higher in the evaporator section as compared to the condenser and adiabatic region. It was observed, the wall temperature was higher than the temperatures of the working fluid in the PHP.

11

Moreover, the temperatures at different turns and region of the PHP were observed to be irregular and changes with time. PHP charged with water presented higher fluid temperature at the evaporator region in comparison to the hybrid nanofluids. This resulted in larger temperature difference between the evaporator and condenser region, therefore contributed to the rise of the thermal resistance when using water as the working fluid. It was demonstrated that the PHP charged with Al2O3-CuO hybrid nanofluid and SiO2-CuO hybrid nanofluid exhibited smaller temperature difference between the evaporator and condenser region analogous to water. This can be explained due to the consideration of

Fig. 12. Temperature contours of PHP at t = 3 s. (a) Al2O3-CuO hybrid nanofluid (b) SiO2-CuO hybrid nanofluid and (c) water. Heating power = 30 W, filling ratio = 50%.

12

M. Zufar et al. / International Journal of Heat and Mass Transfer 146 (2020) 118887

the thermal conductivity enhancement of the hybrid nanofluids in the simulation which have increased the heat transfer capability to absorb and dissipate heat at much greater rate than water. The present numerical results on the temperature contour offer visualization and explanation of the fluid temperature behavior in PHP and were observed to match with the thermal performance exhibited in the present experimental results. In addition, similar temperature behavior in PHP was also observed in other simulation runs tested at 60% filling ratio charged with hybrid nanofluids and water under various heat input. 3.3.2. Two-phase flow behavior and dry out in PHP Fig. 13(a)–(c) depict the two-phase liquid vapor volume fraction contour of PHP charged with Al2O3-CuO hybrid nanofluid, SiO2CuO hybrid nanofluid, and water, respectively under heat input

of 30 W at 50% filling ratio at 3 s time. The vapor and liquid phases in PHP are expressed by the red and blue color, respectively. Investigation on the formation of liquid vapor slug in a time interval accounts as the primary step to understand the complex twophase mechanism in PHP [31]. It was observed that the vapor formations were located at various spots in the PHP for the hybrid nanofluids and water. When PHP was charged with water, only the vapor bubbles can be observed at 3 s simulation time, whereas the vapor slug was not achieved. However, when PHP was charged with the hybrid nanofluids, the vapor bubbles were seen to merge and form into vapor slugs. This phenomenon occurred due to the hybrid nanoparticles dispersion in the base fluid which creates many nucleation sites for the vapor establishment in the walls of PHP. The deficient generation of vapor will lead to tiny amount of heat absorption and removal, henceforth lead to augmentation

Fig. 13. Volume fraction contours of liquid vapor slugs at t = 3 s for (a) Al2O3-CuO hybrid nanofluid (b) SiO2-CuO hybrid nanofluid (c) water in PHP under heat input of 30 W.

Fig. 14. Dry out phenomena in PHP charged with water at (a) 50% and (b) 60% filling ratio under heat input of 100 W.

M. Zufar et al. / International Journal of Heat and Mass Transfer 146 (2020) 118887

of the evaporator temperatures and further enlarge the temperature difference of the evaporator and the condenser region. This has been reported as well by Gunnasegaran et al. [33] who conducted computational means to investigate the formation of the bubbles in a loop heat pipe (LHP) using diamond nanofluid. The author reported that larger vapor formations were observed corresponded to the suspension of nanoparticles in base fluid, therefore the heat transfer was enhanced which resulted in reduction of thermal resistance of the LHP. Moreover, flow of the hybrid nanofluids and water in PHP were initially observed to oscillate in a random manner, however as time progressed the fluid flow was found to be unidirectional with counterclockwise in direction. Simulation runs were conducted for PHP charged with water at 50% and 60% filling ratio under various heat inputs to study the effect of filling ratio on two-phase flow behavior. It was exhibited that dry out phenomena occurred in PHP incorporated with water at 50% filling ratio under heat input of 100 W as shown in Fig. 14 (a), meanwhile, PHP at 60% filling ratio did not experience this dry out condition as shown in Fig. 14(b). Dry out occur due to steep rise in the evaporator temperature due to large heat flux accumulation in PHP, hence urge vapor bubbles and slugs to be generated much faster. Consequently, the fluid flow in PHP changes from vapor bubbles, vapor slugs and finally to annular flow which is characterized as dry out. Dry out is detrimental to the thermal performance of PHP because of high heat accumulation and low dissipation rate. Condensation of the continuous vapor phase could not be achieved in PHP, hence resulted in the rise of the temperature walls and inefficient heat transfer. By incorporating PHP with a higher volume of working fluid or filling ratio, the dry out can possibly be avoided. Though, dry out was more likely to occur when PHP was charged with lower filling ratio. This shows that 60% is the optimal filling to be used in PHP owing to the absence of dry out and achieved the best thermal performance improvement. In the present numerical study, the dry out phenomena did not take place when PHP was charged with hybrid nanofluids at both filling ratio of 50% and 60%. This reveals the potential and capability of hybrid nanofluid to withstand and operate at higher heat flux engineering applications. 3.3.3. Non-linear temperature analysis The non-linear temperature analysis is a suitable method to be used for complex dynamic systems such as PHP two-phase pulsations flow. Pouryoussefi et al. [30] and Kim et al. [44] have incorporated this method to determine the chaotic temperature behavior in PHP. In the present numerical study, the temperature measurement was probed at the specified thermocouple position (T2) as shown in Fig. 2(b). Fig. 15 depicts the temperatures pulsations for Al2O3-CuO hybrid nanofluid, SiO2-CuO hybrid nanofluid, and water at 60% filling ratio under 60 W heating power. From the results, it was displayed that the pulsations and fluctuations of the temperatures at the evaporator section behaved in an irregular and aperiodic manner. Furthermore, the temperatures fluctuations for SiO2-CuO hybrid nanofluid and water were more vigorous and portrayed greater amplitude in comparison to the Al2O3-CuO hybrid nanofluid. It was understood that the thermal conductivity and viscosity properties influenced the pulsations of the respective working fluids, whereby the greatest amplitude was exhibited by SiO2-CuO hybrid nanofluid consecutively water and Al2O3-CuO hybrid nanofluid. The main reason that SiO2-CuO hybrid nanofluid and water were able to pulsate in a greater amplitude because of less shear force acted on the walls of PHP despite of high thermal conductivity of Al2O3-CuO hybrid nanofluid. The temperature pulsations of PHP charged with hybrid nanofluids and water under 60 W heat input at 60% filling ratio exhibited in the present numerical work satisfied with its identical study parameters in the

13

Fig. 15. Temperature pulsations at thermocouple position T2 of PHP charged with hybrid nanofluids and water at 60% filling ratio under heating power of 60 W.

present experimental work, thus, indicates that the present numerical method is reliable. 4. Conclusion The thermal performance of hybrid nanofluids in PHP has been examined experimentally and numerically. The following conclusions can be drawn:  From the experimental results, the hybrid nanofluids exhibited earlier start-up pulsations and required lower heating power for the start-up as compared to water at all the filling ratio tested.  PHP charged with hybrid nanofluids obtained lower average evaporator temperature (Te) as compared to water at heat input above 30 W at all the filling ratio tested. The Te for all the working fluids are almost similar at low heat input below 30 W. Therefore, this highlights the suitability of hybrid nanofluids for higher heating power applications.  SiO2-CuO hybrid nanofluid showed the lowest thermal resistance compared to Al2O3-CuO hybrid nanofluid and water at all the filling ratio owing to its promising thermal conductivity and viscosity properties.  Due to high viscosity, PHP using Al2O3-CuO hybrid nanofluid showed higher thermal resistance than SiO2-CuO hybrid nanofluid despite having greater thermal conductivity. This indicates that the thermal conductivity is not the sole attributes for determination of thermal performance improvement of PHP.  The optimal filling ratio of the four-turns PHP charged with the hybrid nanofluids and water was found to be 60%.  From the simulation results, dry out only occurred for PHP charged with water at 50% filling ratio under 100 W heat input. It is understood that dry out tends to occur when the filing ratio is low. Also, by incorporating hybrid nanofluid in PHP, dry out can be avoided at all filling ratio and heat input values tested in the present study.  The present numerical results showed minimal deviation from the current experimental work and previous experimental studies conducted by other researchers. The simulated evaporator temperature pulsations agree well with those measured; therefore, the simulation method used in the present study is reliable.

14

M. Zufar et al. / International Journal of Heat and Mass Transfer 146 (2020) 118887

Declaration of Competing Interest We assert that we do not have any profitable or associative interest that signifies a conflict of interest correspond to the present work. Acknowledgements This study has been supported by the Universiti Tenaga Nasional (UNITEN) internal grant: UNIIG 2018 (J510050887). Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijheatmasstransfer.2019.118887. References [1] E. Salari, S.M. Peyghambarzadeh, M.M. Sarafraz, F. Hormozi, V. Nikkhah, Thermal behavior of aqueous iron oxide nano-fluid as a coolant on a flat disc heater under the pool boiling condition, Heat Mass Transf. und Stoffuebertragung 53 (1) (2017) 265–275. [2] M.M. Sarafraz, F. Hormozi, S.M. Peyghambarzadeh, Thermal performance and efficiency of a thermosyphon heat pipe working with a biologically ecofriendly nanofluid, Int. Commun. Heat Mass Transf. 57 (2014) 297–303. [3] M.M. Sarafraz, Z. Tian, I. Tlili, S. Kazi, M. Goodarzi, Thermal evaluation of a heat pipe working with n-pentane-acetone and n-pentane-methanol binary mixtures, J. Therm. Anal. Calorim. 2 (2019). [4] M.M. Sarafraz, O. Pourmehran, B. Yang, M. Arjomandi, Assessment of the thermal performance of a thermosyphon heat pipe using zirconia-acetone nanofluids, Renew. Energy 136 (2019) 884–895. [5] A. Selim, Ö. Aç, B. Onur, Experimental investigation on the viscosity characteristics of water based SiO2-graphite hybrid nano fluids 97 (2018) 30–38. [6] S. Suresh, K.P. Venkitaraj, P. Selvakumar, M. Chandrasekar, Synthesis of Al2O3Cu/water hybrid nanofluids using two step method and its thermo physical properties, Colloids Surf. A Physicochem. Eng. Asp. 388 (1–3) (2011) 41–48. [7] M. Hemmat, A. Akbar, A. Arani, M. Rezaie, W. Yan, A. Karimipour, Experimental determination of thermal conductivity and dynamic viscosity of Ag – MgO/ water hybrid nano fluidq, Int. Commun. Heat Mass Transf. 66 (2015) 189–195. [8] A. Selim, G. Yalç, B. Onur, S. Öztuna, Experimental study on the thermal conductivity of water-based CNT-SiO2 hybrid nano fluids 99 (2018) 18–25. [9] S. Khandekar, Thermo-hydrodynamics of Closed Loop Pulsating Heat Pipes, PHD thesis, 2004, p. 154. [10] J. Qu, H. Wu, P. Cheng, X. Wang, Non-linear analyses of temperature oscillations in a closed-loop pulsating heat pipe, Int. J. Heat Mass Transf. 52 (15–16) (2009) 3481–3489. [11] Z. Wan, J. Deng, B. Li, Y. Xu, X. Wang, Y. Tang, Thermal performance of a miniature loop heat pipe using water-copper nanofluid, Appl. Therm. Eng. (2014). [12] M. Kole, T.K. Dey, Thermal performance of screen mesh wick heat pipes using water-based copper nanofluids, Appl. Therm. Eng. 50 (1) (2013) 763–770. [13] Z.H. Liu, Y.Y. Li, R. Bao, Compositive effect of nanoparticle parameter on thermal performance of cylindrical micro-grooved heat pipe using nanofluids, Int. J. Therm. Sci. 50 (4) (2011) 558–568. [14] J. Qu, H. Wu, P. Cheng, Thermal performance of an oscillating heat pipe with Al2O3 – water nano fluids q, Int. Commun. Heat Mass Transf. 37 (2) (2010) 111–115. [15] R.R. Riehl, N. Dos Santos, Water-copper nanofluid application in an open loop pulsating heat pipe, Appl. Therm. Eng. 42 (2012) 6–10. [16] M.R. Tanshen et al., Pressure distribution inside oscillating heat pipe charged with aqueous Al2O3 nanoparticles, MWCNTs and their hybrid, J. Cent. South Univ. 21 (6) (2014) 2341–2348. [17] M. Alhuyi, R. Ghasempour, M. Hossein, ‘‘Experimental investigation of graphene oxide nano fluid on heat transfer enhancement of pulsating heat, pipe” 91 (2018) 90–94. [18] M. Xing, J. Yu, R. Wang, Performance of a vertical closed pulsating heat pipe with hydroxylated MWNTs nanofluid, Int. J. Heat Mass Transf. 112 (2017) 81– 88. [19] Y.H. Lin, S.W. Kang, H.L. Chen, Effect of silver nano-fluid on pulsating heat pipe thermal performance, Appl. Therm. Eng. 28 (11–12) (2008) 1312–1317.

[20] H.R. Goshayeshi, F. Izadi, K. Bashirnezhad, Comparison of heat transfer performance on closed pulsating heat pipe for Fe3O4 and ɤFe2O3 for achieving an empirical correlation, Phys. E Low-Dimensional Syst. Nanostruct. 89 (November 2016) (2017) 43–49. [21] M. Mameli, V. Manno, S. Filippeschi, M. Marengo, Thermal instability of a closed loop pulsating heat pipe: combined effect of orientation and filling ratio, Exp. Therm. Fluid Sci. 59 (2014) 222–229. [22] H.Y. Hsu, M.C. Lin, B. Popovic, C.R. Lin, N.A. Patankar, A numerical investigation of the effect of surface wettability on the boiling curve, PLoS One 12 (11) (2017) e0187175. [23] A. Gandomkar, M.H. Saidi, M.B. Shafii, M. Vandadi, K. Kalan, Visualization and comparative investigations of pulsating ferro-fluid heat pipe, Appl. Therm. Eng. 116 (2017) 56–65. [24] A. Yoon, S.J. Kim, Characteristics of oscillating flow in a micro pulsating heat pipe: fundamental-mode oscillation, Int. J. Heat Mass Transf. 109 (2017) 242– 253. [25] Z.H. Xue, W. Qu, Experimental and theoretical research on a ammonia pulsating heat pipe: new full visualization of flow pattern and operating mechanism study, Int. J. Heat Mass Transf. 106 (2017) 149–166. [26] F. Selimefendigil, H.F. Öztop, MHD Pulsating forced convection of nanofluid over parallel plates with blocks in a channel, Int. J. Mech. Sci. 157–158 (April) (2019) 726–740. [27] F. Selimefendigil, H.F. Öztop, Conjugate mixed convection of nanofluid in a cubic enclosure separated with a conductive plate and having an inner rotating cylinder, Int. J. Heat Mass Transf. 139 (2019) 1000–1017. [28] F. Selimefendigil, H.F. Öztop, Numerical study of MHD mixed convection in a nanofluid filled lid driven square enclosure with a rotating cylinder, Int. J. Heat Mass Transf. 78 (2014) 741–754. [29] F. Selimefendigil, A.J. Chamhka, Natural convection of a hybrid nanofluid-filled triangular annulus with an opening, Comput. Therm. Sci. 8 (6) (2016) 555– 566. [30] S.M. Pouryoussefi, Y. Zhang, Numerical investigation of chaotic flow in a 2D closed-loop pulsating heat pipe, Appl. Therm. Eng. 98 (2016) 617–627. [31] S.M. Pouryoussefi, Y. Zhang, Analysis of chaotic flow in a 2D multi-turn closedloop pulsating heat pipe, Appl. Therm. Eng. 126 (2017) 1069–1076. [32] C.T. O’Sullivan, Newton’s law of cooling—A critical assessment, Am. J. Phys. 58 (10) (1990) 956–960. [33] P. Gunnasegaran, M.Z. Abdullah, M.Z. Yusoff, R. Kanna, Heat transfer in a loop heat pipe using diamond-H2O nanofluid, Heat Transf. Eng. 39 (13–14) (2018) 1117–1131. [34] M.F. Nabil, W.H. Azmi, K. Abdul Hamid, R. Mamat, F.Y. Hagos, An experimental study on the thermal conductivity and dynamic viscosity of TiO2-SiO2 nanofluids in water: ethylene glycol mixture, Int. Commun. Heat Mass Transf. 86 (June) (2017) 181–189. [35] K.A. Hamid, W.H. Azmi, M.F. Nabil, R. Mamat, Experimental investigation of nanoparticle mixture ratios on TiO 2 – SiO 2 nanofluids heat transfer performance under turbulent flow, Int. J. Heat Mass Transf. 118 (2018) 617– 627. [36] N.A. Che Sidik, M. Mahmud Jamil, W.M.A. Aziz Japar, I. Muhammad Adamu, A review on preparation methods, stability and applications of hybrid nanofluids, Renew. Sustain. Energy Rev. 80 (January 2016) (2017) 1112–1122. [37] G. Madalina, G. Huminic, A. Adriana, A. Huminic, Experimental study on thermal conductivity of stabilized Al 2 O 3 and SiO 2 nanofluids and their hybrid, Int. J. Heat Mass Transf. 127 (2018) 450–457. [38] V.M. Patel, Gaurav, H.B. Mehta, Influence of working fluids on startup mechanism and thermal performance of a closed loop pulsating heat pipe, Appl. Therm. Eng. 110 (2017) 1568–1577. [39] H.R. Goshayeshi, M.R. Safaei, M. Goodarzi, M. Dahari, Particle size and type effects on heat transfer enhancement of Ferro-nanofluids in a pulsating heat pipe, Powder Technol. 301 (2016) 1218–1226. [40] M. Alhuyi, R. Ghasempour, M. Hossein, Experimental investigation of graphene oxide nanofluid on heat transfer enhancement of pulsating heat pipe (2018) 91. [41] A.P. Solanki, H. Patel, Experimental analysis of close loop pulsating heat pipe at different filling ratio 2 (2) (2014) 2357–2364. [42] H. Jia, L. Jia, Z. Tan, An experimental investigation on heat transfer performance of nanofluid pulsating heat pipe, J. Therm. Sci. 22 (5) (2013) 484–490. [43] Q. Wu, R. Xu, R. Wang, Y. Li, Effect of C60 nanofluid on the thermal performance of a flat-plate pulsating heat pipe, Int. J. Heat Mass Transf. 100 (2016) 892–898. [44] S. Kim, Y. Zhang, J. Choi, Effects of fluctuations of heating and cooling section temperatures on performance of a pulsating heat pipe, Appl. Therm. Eng. 58 (1–2) (2013) 42–51.