Preliminary experimental study of a bio-inspired, phase-change particle capillary heat exchanger
Introduction
Particulate flow is a multiphase flow found very often in many engineering processes, such as in chemical reactions (fluidized beds), spray-painting, coating (of particles), combustion (fuel-injection), and packaging (of cereals, grains), and in several natural processes as well, for instance in rain fall, river flow (particle sedimentation, erosion), and blood flow (red and white cells flowing with plasma). Usually, the main concern in a particulate flow is the transport of the particulate. Nevertheless, the presence of particulates in a fluid flow can affect the transport of other entities by the flow (e.g., momentum, thermal energy, species, etc.), in which case the particulates can have either a passive or active role in the transport process, enhancing or hindering it.
Consider for instance the use of particles encapsulating (carrying) phase-change material (PCM) and flowing with a cooling fluid through a heat exchanger. In this example, the transport of particles is not the main objective of the transport process; rather, the particles are included in the flow to enhance the convective energy transport by the fluid, mainly in two ways: (1) by storing energy in latent form (an active role) and (2) by “mixing” the flow field (a passive – to energy transport – role). The high energy storage density and small temperature variation during the heat transfer process provided by the encapsulated phase-change material particle has made the first role particularly interesting in recent years to many investigators. For instance, forced convection by micro-encapsulated phase-change materials (MEPCM) slurries, an emerging heat transfer enhancing technology of this decade with potential applications in many fields, such as in cooling military avionics, commercial HVAC systems, and refrigeration heat exchangers, has been investigated theoretically by Charunyakorn et al. [1], Hu and Zhang [2], Roy and Avanic [3], Alisetti and Roy [4], Ho et al. [5], Xin et al. [6], and Wang et al. [7], among others. Hydrodynamic and heat transfer characteristics of slurry containing PCM particles for use as a heat transfer fluid were investigated experimentally as well by Yamagishi et al. [8], Cho and Choi [9], Wang et al. [10] and Zenga et al. [11], among others.
However, the use MEPCM particles in a slurry form for enhancing convection has two main disadvantages, namely: the PCM inside the individual very small particles melt too quickly (because of their minute quantity) when exposed to a high heat flux, locally losing the “small temperature variation” characteristic of latent heating; and, the slurry requires a high pump-power to circulate. The use of small size particles, in relation to the flow channel dimensions, in large quantities (slurry) seems also to dilute the second potential advantage of using particulates as convection enhancers, that of mixing the fluid. Certainly, the small particles do have a mixing effect but the effect is likely to be local (around the particle) and proportional to the particle size. The small particle-to-particle distance of slurry is expected to dump localized flow disturbances generated by the particles as well.
In this study, a different particulate flow is considered for heat convection enhancement, one in which the size of the particles is comparable to the size of the channel flow. This two-phase flow configuration has been inspired by observing the gas exchange process between the alveolar region of the lungs (filled with air) and the blood (containing liquid plasma and red blood cells, RBCs) flowing through an alveolar capillary, Fig. 1. A distinctive characteristic of the alveolar capillary blood flow is the similarity between the diameter of the RBCs and the size of the capillaries (distance between top and bottom capillary membranes). This is one of the key factors believed to be responsible for the high gas transfer efficiency of the lungs [12]. Specifically, it is conceivable that a solid particle with diameter comparable to the channel dimension would act like a broom sweeping along the channel surface, mixing (“breaking”) the boundary layer, reducing the transport resistance, and finally enhancing the convection process.
The conceptual analogy between gas transfer and heat transfer yields naturally the central idea of this study, which is the desire to build a cold plate similar to an alveolar capillary. Thus, a parallel-plates channel mimicking an alveolar capillary, with an isoflux (bottom) surface and an adiabatic (top) surface, such as the one depicted in Fig. 2, is used as a test channel. Particles, with encapsulated phase-change material (EPCM) having diameter similar to the distance between the top and bottom channel surfaces, flow with water through the channel mimicking the RBCs flowing with plasma through a lung capillary. Preliminary test results of investigating the thermal performance of the resulting bio-inspired cold plate are reported here. Specifically, the effects of varying the water flow rate and the particle concentration on the temperature distribution along the heated surface of the channel (and consequently on the surface averaged heat transfer coefficient), are investigated and analyzed for several channel surface heat flux values. We mention in passing a similar concept utilizing a phase-changing particle train as effective cooling technique, flowing with water inside circular tubes, has been considered recently by Ulusarslan and Teke [13], [14], and by Teke and Ulusarslan [15].
Section snippets
Experimental set up
Fig. 3 shows a schematic diagram of the experimental set up, where an inlet reservoir is shown connected to an exit reservoir by the channel test module made of plexy-glass with height H = 5.4 mm and width W = 34 mm (see Fig. 2). The channel has a 45 mm long unheated entrance section (L1), followed by a 121 mm heated section (L2) and a 14 mm unheated exit section (L3), for a total channel length L = L1 + L2 + L3 = 180 mm.
Seven self-adhering electric heaters (Minco HKK5576R45) with sizes 15 mm by 38 mm and
Theoretical considerations
A simplified theoretical analysis is performed considering the two-dimensional schematic of the heated channel as shown in Fig. 5. The thermo-physical properties necessary for the analysis are obtained assuming the particles and the water as constituents of a mixture. In this way, the water–particle mixture can be treated, in a first approximation, as a homogeneous, single-phase constituent. Hence, the particle–fluid effective specific heat, ce, effective density, ρe, and thermal conductivity, k
Experimental results
Fig. 8 shows results of the averaged surface temperature along the heated surface of the channel, , versus the wall heat flux for average flow speed = 0.35 cm/s (which is the lowest speed used here) and for various particle volume-fraction φp. Also included in the same graph are the results for no particle in the channel, a situation equivalent to particle volume-fraction equal to zero. Observe initially, as the particle volume-fraction varies, how the variation for a fixed heat flux
Conclusion
The main conclusion of the preliminary experimental study is that EPCM particles with dimensions similar to the channel dimension, similarly to red blood cells flowing in alveolar capillaries, lower the temperature of the heated surface of a flow channel, and, by consequence, it yields higher heat transfer coefficient than the heat transfer coefficient obtained when the flow is clear of particles. Moreover, this increase in heat transfer efficiency can be linked to two effects, namely the extra
References (21)
- et al.
Microencapsulated phase change material slurries flow in circular ducts
Int. J. Heat Mass Transfer
(1991) - et al.
Laminar forced convection heat transfer with phase change material suspensions
Int. Commun. Heat Mass Transfer
(2001) - et al.
Flow and heat transfer be behaviors of phase change material slurries in a horizontal circular tube
J. Heat Mass Transfer
(2007) - et al.
Thermal characteristic of paraffin in a spherical capsule during freezing and melting processes
Int. J. Heat Mass Transfer
(2000) - et al.
An experimental investigation of the capsule velocity, concentration rate and the spacing between the capsules for spherical capsule train flow in a horizontal circular pipe
Powder Technol.
(2005) - et al.
An experimental determination of pressure drops in the flow of low density spherical capsule train inside horizontal pipes
Exp. Therm. Fluid Sci.
(2006) - et al.
Mathematical expression of pressure gradient in the flow of spherical capsules less dense than water
Int. J. Multiphase Flow
(2007) - et al.
Heat transfer of solid–liquid phase change material suspension in circular pipes: effects of wall conduction
Numer. Heat Transfer A
(2004) - et al.
Forced convection heat transfer to phase change material slurries in circular ducts
J. Thermophys. Heat Transfer
(2000) - et al.
Heat transfer of solid–liquid phase change material suspension in circular pipes: effects of wall conduction
Numer. Heat Transfer A
(2004)