Pressure drop and convective heat transfer of Al2O3/water and MWCNT/water nanofluids in a chevron plate heat exchanger
- a Department of Energy Engineering, Co-Innovation Center for Advanced Aero-Engine, Zhejiang University, Hangzhou 310027, China
- b Department of Energy Sciences, Lund University, P.O. Box 118, Lund SE-22100, Sweden
- Received 27 February 2015, Revised 11 April 2015, Accepted 19 May 2015, Available online 9 June 2015
Highlights
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Thermal performance of nanofluids in a brazed plate heat exchanger.
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Experiments on heat transfer and pressure drop.
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Revealed importance of presenting data.
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No anomalous heat transfer enhancement.
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No effects of Brownian motion or thermophoresis.
Abstract
Heat transfer and pressure drop characteristics of Al2O3/water and MWCNT/water nanofluids flowing in a chevron-type plate heat exchanger were experimentally investigated and compared with those of water. Results showed that heat transfer seemed to be improved by using nanofluids at constant Reynolds number. However, little heat transfer enhancement was observed based on a constant flow velocity. The heat transfer deterioration of MWCNT/water nanofluids was more intensive than Al2O3/water nanofluids due to the relatively large viscosity increase of MWCNT/water nanofluids. A new heat transfer correlation was proposed based on the experimental data of water and it predicts the experimental data of nanofluids accurately when the measured nanofluid properties (thermal conductivity and viscosity) were adopted for calculation. Besides, the pressure drop of nanofluid was reasonably higher than that of water and seemed to increase with increasing particle volume concentrations due to the increase in viscosity. However, there was not much difference between the pressure drop of nanofluids and that of water at low particle volume concentrations. A correlation for predicting the friction factor was obtained and it fitted the experimental data very well.
Keywords
- Heat transfer;
- Pressure drop;
- Plate heat exchanger;
- Al2O3/water nanofluid;
- MWCNT/water nanofluid
Nomenclature
- A
total heat transfer area, m2
- bc
half of the plate depth, m
- cp
specific heat, J/(kg K)
- D
diameter, m
- f
friction factor, dimensionless
- HTC, h
heat transfer coefficient, W/(m2 K)
- L
length, m
- LMTD
log mean temperature difference, K
- m
mass flow rate, kg/s
- N
number of channels, dimensionless
- Nu
Nusselt number, dimensionless
- ΔP
pressure drop, Pa
- PHE
plate heat exchanger
- Pr
Prandtl number, dimensionless
- Q
heat, W
- Re
Reynolds number, dimensionless
- T
temperature, K
- U
total heat transfer coefficient, W/(m2 K)
- u
velocity, m/s
- V
volumetric flow rate, litre/s
- w
plate width, m
Greek symbols
- λ
thermal conductivity, W/(m K)
- μ
dynamic viscosity, Pa s
- ρ
density, kg/m3
- δ
plate thickness, m
- Φ
surface enlargement ratio, dimensionless
- ϕ
volume concentration, vol.%
Subscripts
- ave
average
- b
base fluid
- c
cold
- e
equivalent
- exp
experimental
- h
hot
- i
inlet
- nf
nanofluid
- o
outlet
- p
nanoparticle
- plate
plate
- w
water
1. Introduction
With the development of thermal engineering and industrial intensification, more efficient and compact heat transfer systems are needed. Therefore, many efforts have been devoted to improving the heat transfer equipment design and enhancing the heat transfer performance of working fluids [1]. The plate heat exchanger (PHE) is widely used in many applications including food processing, heating and cooling applications and chemical industry for its high efficiency (high heat transfer coefficient) and compactness (low volume/surface ratio) [2] and [3]. The flow inside the narrow PHE channels may separate and reattach successively, creating strong turbulence and thus enhancing the heat transfer. However, the complexity caused by the modulated surface of PHEs may significantly increase the pressure drop, which is undesirable in practical applications [4]. Therefore, it is necessary to investigate and evaluate the counteracting effect between the increased heat transfer and increased pressure drop in PHEs.
On the other hand, the thermal performance of working fluids is also a controlling factor in improving efficiency of heat transfer systems [5]. Because nanofluids generally have high thermal conductivities, they could be adopted as the working fluids and might enhance heat transfer. Nanofluids, which are expected to increase the heat transfer coefficient (HTC) with little pressure drop penalty, have received considerable scientific interest during the last decade [6], [7], [8], [9], [10], [11] and [12]. Nguyen et al. [6] experimentally investigated the heat transfer characteristics of Al2O3/water nanofluid in a liquid cooling system and found that the HTC increased by 40% at a volume concentration of 6.8%. Putra et al. [7] experimentally studied the convective heat transfer of CuO and Al2O3 nanofluids flowing in a horizontal tube and observed heat transfer deterioration. They suggested the deterioration was caused by nanoparticle deposition and particle/fluid slip. Wu et al. [8] investigated the heat transfer characteristics of Al2O3/water nanofluids flowing in a double-pipe helical heat exchanger. They found that heat transfer was enhanced compared to the base fluid at constant Reynolds number. However, little heat transfer enhancement was observed based on the constant flow velocity. The above discussion showed that comparable results from different groups vary widely. Therefore, further research on convective heat transfer of nanofluids is necessary.
Previous experimental work of nanofluids mainly focused on simple flow geometries [7], [13] and [14], while there are limited investigations on the thermal performance of nanofluids in complex or enhanced geometries [4], [2], [15], [16] and [17]. Pantzali et al. [4] investigated the effects of CuO/water nanofluid as coolants in a miniature PHE experimentally and numerically. They found that less nanofluid flow rates were required at a given heat load and thus the pressure drop of nanofluid was lower than that of water. Pandey et al. [2] experimentally observed higher HTCs of Al2O3/water nanofluids than water in a corrugated PHE. The HTC increased with increasing Reynolds number and Peclet number as well as with decreasing nanoparticle volume concentrations. The pressure drop of nanofluids increased with increasing nanofluid volume concentrations and was higher than that of water.
On the basis of the open literatures, it is found that heat transfer is usually enhanced by using nanofluids when laminar flow is encountered in simple geometries (i.e., circular tubes). However, the heat transfer performance of nanofluids under turbulent flow is inconsistent. Thus the authors are motivated to investigate the effect of using nanofluids in complex geometries. The heat transfer and pressure drop characteristics of Al2O3/water and MWCNT/water nanofluids in a chevron PHE were experimentally investigated in this paper to explore the possibility and efficacy of using nanofluids in compact heat transfer systems. The results of nanofluids were compared with those of water. Experimentally measured thermo-physical properties of nanofluids (i.e., viscosity and thermal conductivity) were used in this work. Possible pressure drop and heat transfer correlations were proposed for water and nanofluids flowing in PHEs.
