Tranquil rhymes with:
coequal, equal, sequel, unequal, actual, aggarwal, agrawal, bilingual, casual, coequal, cromwell, dingwall, equal, heterosexual, homosexual, individual, monolingual, multilingual, mutual, perceptual, semiannual, sendawal, sensual, sequel, spiritual, textual, tidwell, unequal, unusual, usual, visual
Tranquil sounds like:
transsexual, trenkle, triangle, tringali, trinkle, tyrannical