Thermal Performance of Pulsating Heat Stripes Built With Plastic Materials

Abstract

A low-cost, flexible pulsating heat pipe (PHP) was built in a composite polypropylene sheet consisting of three layers joint together by selective laser welding, to address the demand of heat transfer devices characterized by low weight, small unit thickness, low cost, and high mechanical flexibility. A thin, flexible, and lightweight heat pipe is advantageous for various aerospace, aircraft, and portable electronic applications where the device weight, and its mechanical flexibility are essential. The concept is to sandwich a serpentine channel, cut out in a polypropylene sheet and containing a self-propelled mixture of a working fluid with its vapor, between two transparent sheets of the same material; this results into a thin, flat enclosure with parallel channels hence the name “pulsating heat stripes” (PHS). The transient and steady-state thermal response of the device was characterized for different heat input levels and different configurations, either straight or bent at different angles. The equivalent thermal resistance was estimated by measuring the wall temperatures at both the evaporator and the condenser, showing a multifold increase of the equivalent thermal conductance with respect to solid polypropylene.

Characterization of polypropylene pulsating heat stripes: Effects of orientation, heat transfer fluid, and loop geometry

“…In the case of FC-72 and ethanol, and inclinations of 45 • and vertical, the measured values of the equivalent thermal resistance are identical within the experimental error, both during the ascending and during the descending heating power supply ramps. In these cases, the PHP/PHS attain the overall minimum equivalent thermal resistance for a heating power supply of about 30 W, which has a magnitude four to five times smaller than the equivalent thermal resistance a composite polypropylene sheet having the same composition, size, thickness and weight, in agreement with previous results [8,11,12].…”

“…First, the serpentine channel was cut-out in a black sheet (0.7 mm thickness) using a commercial laser cutter (HPC Laser LS1290 Pro). Then, the channel was sandwiched between two transparent sheets (0.4 mm thickness each), which were tightly bonded to the black sheet by selective transmission laser welding using a nanosecond pulsed fibre laser (SPI Lasers G4 HS-L 20W) [7,8,11,12]. The resulting composite polypropylene sheet had an overall thickness of 1.5 mm.…”

“…If the channel width is large, the thin (0.4 mm) polypropylene wall changes its shape according to the pressure difference between the heat transfer fluid and the atmosphere. This affects both the pressure level during operation and the hydraulic diameter, as shown by preliminary experiments conducted on a polypropylene PHP/PHS having a channel width of 9 mm [8].…”

“…The pressure measured inside the serpentine channel (Figure 7) qualitatively mirrors the trend observed for temperature measurements, although it displays a somewhat smoother response to step changes in the heating power supply. At the end of each test (2.5 to 5 hours), one can observe a small increase of pressure with respect to the beginning, which could be due to (i) gas permeation through the polymer channel walls, (ii) potential imperfections in manufacturing (iii) micro-leaks in the connection with the valve and the pressure sensor, (iv) the viscoelastic relaxation of the polymer channel walls [8]. Whilst this minor pressure increase does not affect significantly the device performance within the duration of a single test, it represents an obvious issue over long periods of time.…”

“…To address the demand of heat transfer devices characterized by low weight, small unit thickness, low cost, and high mechanical flexibility, it was recently proposed to embed the PHP serpentine channel into a composite polypropylene sheet [7,8]. The concept is to cut out the serpentine channel in a black polypropylene sheet, sandwiched between two transparent sheets of the same material and bonded together by selective transmission laser welding [9,10].…”